Polypropylene compositions having good transparency and improved impact resistance

Disclosed is a polymer composition containing: PA1 (A) from 70 to 98 parts by weight of a crystalline propylene homopolymer, or a crystalline random copolymer of propylene with ethylene and/or C.sub.4 -C.sub.10 .alpha.-olefins, containing from 0.5 to 10% by weight of ethylene and/or .alpha.-olefins [fraction (A)]; and PA1 (B) from 2 to 30 parts by weight of elastomeric copolymer of ethylene with one or more C.sub.4 -C.sub.10 .alpha.-olefins, containing from 60 to 85% by weight of ethylene, and partially soluble in xylene at 25.degree. C. [fraction (B)]; wherein the composition has an intrinsic viscosity value in tetrahydronaphthalene at 135.degree. C. of the fraction soluble in xylene at 25.degree. C. ranging from 0.8 to 1.1 dl/g.

The present invention relates to polypropylene compositions having improved 
impact resistance characteristics and good transparency. 
It is known that for certain applications it is useful to decrease the 
crystallinity of the propylene homopolymer by copolymerization of the 
propylene with small quantities of ethylene and/or .alpha.-olefins such as 
1-butene, 1-pentene and 1-hexene. In this manner one obtains the so called 
random crystalline propylene copolymers which, when compared to the 
homopolymer, are essentially characterized by better flexibility and 
transparency. 
These materials can be used in many application sectors, such as, for 
example irrigation pipes, pipes for transporting drinking water and other 
liquid food, heating equipments, single layer bottles (for detergents), 
multilayer bottles (for beverages and perfumes), single layer or 
multilayer film for various packaging and rigid food containers. 
Propylene random copolymers, however, although they have good transparency, 
do not offer, especially at low temperatures, sufficiently better impact 
resistance than the homopolymer which can be satisfactory used for the 
applications listed above. 
It has been known for a long time that the impact resistance of the 
polypropylene can be improved by adding an adequate quantity of 
elastomeric propylene-ethylene copolymer to the homopolymers by mechanical 
blending or sequential polymerization. However, this improvement is 
obtained at the expenses of the transparency of the material. 
To avoid this inconvenience, U.S. Pat. No. 4,634,740 suggests the blending 
of the polypropylene, in the molten state, with propylene-ethylene 
copolymers obtained with specific catalysts, and having an ethylene 
content ranging from 70 to 85% by weight. However, the preparation of such 
compositions requires the separate synthesis of the homopolymer and the 
copolymer, and their subsequent blending. This clearly presents a 
disadvantage in terms of the investment and production cost involved in 
producing such material. Moreover, said compositions present transparency 
values (Haze) substantially comparable to those of the propylene 
homopolymer. Said patent, therefore, does not teach how to obtain 
compositions having good transparency. 
A further disadvantage of the compositions described in the above mentioned 
U.S. patent is that the propylene-ethylene copolymer is synthesized with 
catalysts which lack a sufficiently high catalytic activity to avoid the 
purification process. 
In order to overcome the above mentioned disadvantages the Applicant has 
previously produced transparent polypropylene compositions offering impact 
resistance at low temperatures, which can be prepared directly in 
polymerization (sequential copolymerization). Said compositions, which 
constitutes the subject of published patent application EP-A-373660, 
comprise both a crystalline random propylene copolymer, and an elastomeric 
copolymer of ethylene with propylene and/or C.sub.4 -C.sub.8 
.alpha.-olefins containing from 20 to 70% by weight of ethylene. According 
to said patent application, the compositions mentioned above have a good 
balance of mechanical and chemical-physical properties due to the fact 
that the value of the content of ethylene of the elastomeric copolymer 
multiplied by the value of the ratio between the intrinsic viscosity 
(I.V.) of the elastomeric copolymer soluble in xylene at ambient 
temperature, and the one of the propylene random copolymer is comprised 
within a predetermined range. However, the values of the Izod impact 
resistance at 0.degree. C. and the ductile/fragile transition temperature 
render said compositions inadequate for use at low temperatures, such as 
in the case of food preservation. 
Published European patent application EP-A-0557953, in the name of the 
Applicant, describes polyolefin compositions where one obtains a good 
balance of transparency, stiffness, and impact resistance even at low 
temperatures, by modifying a crystalline random copolymer of propylene 
with the proper quantities of a mechanical mixture comprising an 
elastomeric copolymer and one or more polymers chosen from LLDPE, LDPE and 
HDPE. 
New polypropylene compositions have now been found which have an optimum 
balance of transparency, stiffness and impact resistance even at low 
temperatures. 
Accordingly, the present invention provides polymer compositions 
comprising: 
(A) from 70 to 98 parts by weight of a crystalline propylene homopolymer, 
or a crystalline random copolymer of propylene with ethylene and/or 
C.sub.4 -C.sub.10 .alpha.-olefins, containing from 0.5 to 10% by weight of 
ethylene and/or .alpha.-olefins [fraction (A)]; and 
(B) from 2 to 30 parts by weight of elastomeric copolymer of ethylene with 
one or more C.sub.4 -C.sub.10 .alpha.-olefins, containing from 60 to 85% 
by weight of ethylene, and partially soluble in xylene at 25.degree. C. 
[fraction (B)]; wherein 
said composition has an intrinsic viscosity value in tetrahydronaphthalene 
at 135.degree. C. of the fraction soluble in xylene at 25.degree. C. 
ranging from 0.8 to 1.1 dl/g. 
The preferred polymer compositions are those with an intrinsic viscosity 
value ranging from 0.9 to 1.1 dl/g, limits included. 
Also preferred are the polymer compositions where fraction (A) constitutes 
75 to 85 parts by weight of the polymer composition, while fraction (B) 
constitutes 15 to 25 parts by weight of said composition. 
The preferred quantity of ethylene and/or C.sub.4 -C.sub.10 .alpha.-olefin 
present in the copolymer of fraction (A) ranges from 1 to 5% by weight. 
The quantity of ethylene present in the copolymer of fraction (B) 
preferably is from 63 to 85% by weight, most preferably from 65 to 75%. 
Examples of C.sub.4 -C.sub.10 .alpha.-olefins that can be used as 
comonomers in fractions (A) and (B) are 1-butene, 1-pentene, 1-hexene and 
4-methyl-1-pentene. Particularly preferred is 1-butene. 
The polymer compositions of the present invention have MFR values (ASTM D 
1238 L) preferably ranging from 7 to 20 g/10 min. 
Moreover, generally said compositions have flexural modulus values ranging 
from 500 to 1600 MPa, and ductile/fragile transition temperatures ranging 
from +10.degree. to -50.degree. C. 
In order to obtain improved transparency and stiffness, the polymer 
compositions of the present invention may optionally be nucleated with 
substances commonly used for this purpose, such as dibutylidenesorbitol 
(DBS). The nucleating agents are preferably added in quantities ranging 
from 1000 to 3000 ppm. 
The compositions of the present invention can be prepared by sequential 
copolymerization of the monomers in the presence of stereospecific 
Ziegler-Natta catalysts supported on magnesium dihalides. 
The polymerization is carried out in at least two steps: in the first 
stage, one carries out the synthesis of the polymer of fraction (A), in 
the second one, the synthesis of the polymer of fraction (B). The 
synthesis of the latter occurs in the presence of the polymer obtained and 
the catalyst used in the preceding stage. 
The polymerization process may be done in a continuous or batch manner, 
following known techniques, operating in liquid phase [for fraction (A)], 
optionally in the presence of an inert diluent, or in a gaseous phase, or 
with mixed liquid-gas techniques. Preferably the polymerization is 
conducted in gas phase. The polymerization of fraction (B) is conducted in 
gas phase. 
Reaction times and temperatures relative to the two steps are not critical 
and are advantageously in the range from 0.5 to 5 hrs, and from 50.degree. 
C. to 90.degree. C. respectively. Regulation of the molecular weight is 
done by using molecular weight regulators commonly used, e.g. hydrogen and 
ZnEt.sub.2. 
The aforementioned intrinsic viscosity values of the fraction soluble in 
xylene at 25.degree. C. which characterizes the compositions of this 
invention, are obtained by using higher amounts of the molecular weight 
regulator. When fraction (B) is synthesized using hydrogen as the 
molecular weight regulator, the molar ratio of hydrogen to ethylene is 
from 0.7 to 1.0. 
The catalysts that can be used to produce the polymer compositions of the 
present invention are well known in patent literature. Particularly suited 
are the catalysts described in U.S. Pat. No. 4,339,054 and European Patent 
No. 45977. Other examples of catalysts are described in U.S. Pat. No. 
4,472,524 and 4,473,660. 
The above mentioned stereospecific catalysts used in the polymerization 
comprise the product of the reaction between: 
a) a solid component, containing a titanium compound and an electron-donor 
compound (internal electron-donor) supported on magnesium chloride, 
b) an aluminum alkyl compound (cocatalyst) and 
c) an electron-donor compound (external electron-donor). 
Said catalysts are preferably capable of producing homopolymer 
polypropylene having an isotactic index higher than 90%. 
The solid catalyst component (a) contains as electron-donor a compound 
selected among the ethers, ketones, lactones, compounds containing N, P 
and/or S atoms, and mono- and dicarboxylic acid esters. 
Particularly suited are phthalic acid esters such as diisobutyl, dioctyl 
and diphenyl phthalate, monobenzyl monobutyl phthalate; malonic acid 
esters such as diisobutyl and diethyl malonate; alkyl and arylpivalates; 
alkyl, cycloalkyl and aryl meleates; alkyl and aryl carbonates such as 
diisobutyl carbonate, monoethyl monophenyl carbonate, and diphenyl 
carbonate; succinic acid esters such as mono- and diethyl succinate. Other 
electron-donors particularly suited are the 1,3-diethers of formula (I), 
##STR1## 
wherein R.sup.I and R.sup.II, are the same or different and represent 
C.sub.1 -C.sub.18 alkyl, C.sub.3 -C.sub.18 cycloalkyl, or C.sub.6 
-C.sub.18 aryl radicals; R.sup.III and R.sup.IV, are the same or different 
and are alkyl radicals with 1 to 4 carbon atoms. 
The ethers of the type described are illustrated in published European 
patent application EP-A-361 493. 
Examples representative of ethers of formula (I) are 
2-methyl-2-isopropyl-1,3-dimethoxypropane, 
2,2-diisobutyl-1,3-dimethoxypropane, and 
2-isopropyl-2-cyclopentyl-1,3-dimethoxypropane. 
The preparation of the described catalyst components is done according to 
various methods. One of them consists of milling or co-milling the 
magnesium dihalide (used in the anhydrous state containing less than 1% 
water), together with the titanium compound, and the electron-donor 
compound under conditions where the magnesium dihalide is activated; the 
milled product is then treated one or more times with excess TiCl.sub.4 at 
temperatures from 80.degree. to 135.degree. C., and subsequently washed 
repeatedly with a hydrocarbon (hexane, for example) until the chlorine 
ions have disappeared from the wash waters. 
According to another method the anhydrous magnesium halide is preactivated 
following known methods, and then reacted with excess TiCl.sub.4 
containing the electron-donor compound in solution. In this case the 
operation also takes place at a temperature from 80.degree. to 135.degree. 
C. Optionally the TiCl.sub.4 treatment is repeated. The solid is then 
washed with hexane or other solvents to eliminate all traces of unreacted 
TiCl.sub.4. 
According to another method, a MgCl.sub.2 nROH adduct (particularly in the 
form of spherical particles) where n is generally a number ranging from 1 
to 3 and ROH is ethanol, butanol or isobutanol, is reacted with excess 
TiCl.sub.4 containing the electron-donor compound in solution. The 
reaction temperature generally ranges from 80 to 120.degree. C. After the 
reaction the solid is isolated and reacted one or more times with 
TiCl.sub.4, and then washed with a hydrocarbon solvent until all traces of 
unreacted TiCl.sub.4 have been eliminated. 
According to yet another method, magnesium alcoholates and 
chloroalcoholates (the chloroalcoholates can be prepared according to U.S. 
Pat. No. 4,220,554) are reacted with excess TiCl.sub.4 containing the 
electron-donor compound in solution, operating under the same conditions 
already described. 
The titanium compound in the solid catalyst component, expressed as Ti 
content, is generally present in the amount ranging from 0.5 to 10% by 
weight, and the quantity of the electron-donor compound that remains set 
on the solid (internal donor) usually ranges from 5 to 20% in moles with 
respect to the magnesium dihalide. 
Titanium compounds which can be used for the preparation of catalyst 
components are halides or halogen alcoholates. Titanium tetrachloride is 
the preferred compound. Satisfactory results are obtained also with 
titanium trihalides, particularly TiCl.sub.3 HR (HR=Hydrogen Reduced), 
TiCl.sub.4 ARA (ARA=Aluminum Reduced and Activated), and with titanium 
halide alcoholates such as TiCl.sub.3 OR, where R is a phenyl radical. 
The preparations indicated above lead to the formation of activated 
magnesium dihalide. Besides the ones already mentioned, other reactions 
are known in the art which lead to the formation of activated magnesium 
dihalides starting from magnesium compounds which are different from the 
magnesium halides, such as magnesium carboxylates. 
The active form of magnesium halides in the solid catalyst component can be 
recognized by the fact that in the X-ray spectrum of the catalyst 
component the major intensity reflection presents a width at half-peak at 
least greater than 30% with respect to the major intensity reflection 
which appears in the spectrum of the nonactivated magnesium dihalide, or 
by the fact that the major intensity reflection (which appears in the 
spectrum of the nonactivated magnesium halides, having a surface area 
smaller than 3 m.sup.2 /g) is absent and in its place there is a halo with 
the maximum intensity shifted with respect to the position of the maximum 
intensity reflection of the nonactivated magnesium dihalide. The most 
active forms of magnesium halide are those where the X-ray spectrum shows 
a halo. 
Among the magnesium dihalides, the magnesium chloride is the preferred 
compound. In the case of the most active forms of magnesium chloride, the 
X-ray spectrum of the catalyst component shows a halo instead of the 
reflection, which in the spectrum of the nonactivated magnesium chloride 
is situated at the distance of 2.56 .ANG.. 
As cocatalysts (b), one preferably uses the trialkyl aluminum compounds, 
such as Al-triethyl, Al-triisobutyl and Al-tri-n-butyl. Other examples of 
cocatalysts (b) are the linear or cyclic Al-alkyl compounds containing two 
or more Al atoms bonded by means of O, or N atoms, or by SO.sub.2, 
SO.sub.3 or SO.sub.4 groups. Some examples of these compounds are: 
(C.sub.2 H.sub.5).sub.2 --Al--O--Al(C.sub.2 H.sub.5).sub.2 
(C.sub.2 H.sub.5).sub.2 --Al--N(C.sub.6 H.sub.5)--Al(C.sub.2 H.sub.5).sub.2 
(C.sub.2 H.sub.5).sub.2 --Al--SO.sub.2 --Al--(C.sub.2 H.sub.5).sub.2 
CH.sub.3 --[(CH.sub.3)Al--O--].sub.n --Al(CH.sub.3).sub.2 
--[(CH.sub.3)Al--O].sub.n -- 
wherein n is a number from 1 to 20. 
In general, the Al-Alkyl compound is present in quantities that allow the 
Al/Ti ratio to vary from 1 to 1000. 
The electron-donor compounds (c) that can be used as external 
electron-donors (added to the Al-alkyl compound) comprise the aromatic 
acid esters (such as alkylic benzoates), heterocyclic compounds (such as 
the 2,2,6,6-tetramethylpiperidine and the 2,6-diisopropylpiperidine), and 
in particular silicon compounds containing at least one Si--OR bond (where 
R is a hydrocarbon radical). Some examples of silicon compounds are: 
(tert-C.sub.4 H.sub.9).sub.2 Si(OCH.sub.3).sub.2, (C.sub.6 H.sub.5).sub.2 
Si(OCH.sub.3).sub.2 and (C.sub.6 H.sub.5).sub.2 Si(OCH.sub.3).sub.2. The 
1,3-diethers of formula (I) are also suitable to be used as external 
donors. In the case that the internal donor is one of the 1,3-diethers of 
formula (I), the external donor can be omitted. 
The catalysts can be precontacted with small quantities of olefins 
(prepolymerization), maintaining the catalyst in suspension in a 
hydrocarbon solvent, and polymerizing at temperatures ranging from ambient 
to 60.degree. C. The quantity of polymer produced is from 0.5 to 3 times 
the weight of the catalyst. 
The prepolymerization can also be carried out in liquid propylene under the 
temperature conditions indicated above, and can produce quantities of 
polymer that can reach up to 1000 g per gram of catalyst component. 
The data reported in the examples relative to the composition and 
properties of the polymer compositions have been determined by way of the 
following methods: 
Intrinsic viscosity: in tetrahydronaphthalene at 135.degree. C. 
MFR: according to ASTM D-1238, condition L. 
Ethylene content (C.sub.2): IR spectroscopy. 
Fractions soluble and insoluble in xylene: dissolving a sample of the 
material in xylene at 125.degree. C. and allowing the solution to cool to 
ambient temperature. The soluble and insoluble fractions are separated by 
filtration. 
Flexural modulus: according to ASTM D-790 (tangent). 
Impact resistance (Izod): according to ASTM D-256 (notched specimen). 
Ductile/fragile transition temperature: by internal Himont method, where 
the ductile/fragile transition is defined as the temperature at which 50% 
of the specimens present fragile cracks when subjected to the impact of a 
ram having a predetermined weight and falling from a given height. 
Haze: according to ASTM D-1003 on 1 mm thick specimens. 
Melting point: by way of DSC. 
Elongation at yield: according to ASTM D-638. 
Elongation at break: according to ASTM D-638. 
VICAT: according to ASTM D-1525.

The following examples are given in order to illustrate and not limit the 
present invention. 
Preparation of catalyst component (a) 
The solid catalyst component used in the examples is prepared as follows. 
In inert atmosphere are introduced, into a reactor equipped with agitator, 
28.4 g of MgCl.sub.2, 49.5 g of anhydrous ethanol, 100 ml of ROL OB/30 
vaseline oil, 100 ml of silicone oil with a viscosity of 350 cs, and the 
content is then heated to 120.degree. C. until the MgCl.sub.2 is 
dissolved. The hot reaction mixture is then transferred to a 1500 ml 
vessel equipped with a T-45 N Ultra Turrax agitator, said vessel 
containing 150 ml of vaseline oil and 150 ml of silicon oil. The 
temperature is maintained at 120.degree. C., while the agitation continues 
for 3 minutes at 3000 rpm. The mixture is then discharged to a 2 liter 
vessel equipped with agitator and containing 1000 ml of anhydrous 
n-heptane cooled to 0.degree. C. The particles obtained are recovered by 
filtration, washed with 500 ml of n-hexane, and the temperature is 
gradually increased from 30 to 180.degree. C. in nitrogen flow until an 
MgCl.sub.2.2,1C.sub.2 H.sub.5 OH adduct is obtained. 
25 g of the adduct are transferred to a reactor equipped with agitator and 
containing 625 ml of TiCl.sub.4, at 0.degree. C. and under agitation. The 
temperature is brought to 100.degree. C. and the mixture is heated at that 
temperature for one hour. When the temperature reaches the 40.degree. C., 
one adds diisobutyl phthalate in such a quantity that the magnesium molar 
ratio with respect to the phthalate is 8. 
The content of the reactor is heated to 100.degree. C. for 2 hours under 
agitation, and then the solid is allowed to settle, after which the liquid 
is syphoned while still hot. 550 ml of TiCl.sub.4 are added, and the 
mixture is heated to 120.degree. C. for one hour under agitation. The 
agitation is interrupted, the solid is allowed to settle, and the liquid 
is syphoned while still hot. The solid is washed 6 times with 200 ml of 
n-hexane each time at 60.degree. C., and then 3 times at room temperature. 
Polymerization 
The polymerization is carried out in continuous in a series of reactors 
equipped with devices to transfer the product from one reactor to the one 
immediately next to it. 
In the following examples the polymerization process is preceded by 
prepolymerization, which is carried out in a reactor in the presence of an 
excess of liquid propylene, said prepolymerization lasting from about 1.5 
to about 2 minutes, and at a temperature ranging from 20 to 24.degree. C. 
The prepolymer is then transferred to the first reactor where the 
polymerization takes place in gas phase in order to form fraction (A). 
The polymer of fraction (A) is fed from the first to the second reactor 
after all the unreacted monomers have been eliminated. Fraction (B) is 
formed in this reactor. 
In the prepolymerization and polymerization examples are used, together 
with solid catalyst component (a) (prepared as described above), the 
triethylaluminum (TEAL), as cocatalyst, and the 
dicyclopentyl-dimethoxysilane (DCPMS) as external electron-donor. The 
weight ratios between TEAL and DCPMS, and TEAL and Ti are reported in 
Table 1. 
Table 2 shows the temperature, pressure and molar ratios of the monomers 
introduced in the single reaction stages. 
In gas-phase the hydrogen and monomers are continuously analyzed by gas 
chromatography, and the desired concentrations are maintained constant 
through proper feeding. 
During the course of the single polymerization steps the temperature and 
the pressure are maintained constant. 
Tables 3 and 4 show the composition characteristics and the properties of 
the products of the examples according to the invention (Examples 1-3) and 
the comparative example (1c). 
TABLE 1 
______________________________________ 
Example 1 2 3 4 
______________________________________ 
TEAL/DCPMS 5.4 3.6 5.8 4.0 
TEAL/Ti 6.2 7.2 7.7 -- 
______________________________________ 
TABLE 2 
______________________________________ 
Examples 1 2 3 1c 
______________________________________ 
1st gas phase reactor 
Temperature (.degree.C.) 
75 75 75 75 
Pressure (bar) 
22.5 22.5 24.0 17.0 
Residence time (min) 
65 64 69 -- 
H.sub.2 /C.sub.3 (moles) 
0.02 0.02 -- 0.035 
C.sub.2 /(C.sub.2 + C.sub.3) (moles) 
0.01 0.005 0.006 -- 
2nd gas phase reactor 
Temperature (.degree.C.) 
70 70 70 70 
Pressure (bar) 
12 12 15 11.5 
Residence time (min) 
29 25 53 -- 
H.sub.2 /C.sub.2 (moles) 
0.8 0.81 0.8 0.5 
C.sub.4 /(C.sub.4 + C.sub.2) (moles) 
0.5 0.5 0.5 0.5 
______________________________________ 
TABLE 3 
______________________________________ 
Example 1 2 3 1c 
______________________________________ 
Fraction (A) (parts by wt.) 
81.1 80.0 74.4 75 
C.sub.2 in Fraction (A) 
2.0 1.0 1.3 45 
(% by weight) 
Solubility in xylene of 
4.4 3.2 5.9 4.5 
Fraction (A) (parts by wt.) 
C.sub.2 in Fraction (B) 
70.0 70.0 65.0 65 
(% by weight) 
I.V. of fraction soluble in 
0.93 0.96 0.93 1.28 
xylene 
______________________________________ 
TABLE 4 
______________________________________ 
Examples 1 2 3 1c 
______________________________________ 
MFR "L" (g/10 min) 
9.0-9.5 12-13 9-10 12 
Flex. modulus (MPa) 
900-950 1050-1150 800-850 
890 
IZOD at 23.degree. C. (J/m) 
410 250 600 580 
IZOD at 0.degree. C. (J/m) 
70 45 480 -- 
Ductile/Fragile 
-27.0 -26.0 -40.0 -42 
transition temp. (.degree.C.) 
Yield stress (MPa) 
24.0 27.0 20.6 21 
Elongation at break 
500 500 500 500 
(%) 
Melt temp. (.degree.C.) 
153 158 154 154 
VICAT (.degree.C.) 
130.0 137.0 128.0 
128 
Haze.sup.1 (%) 
16 20 18 31 
______________________________________ 
.sup.1 values obtained after the addition of DBS (2500 ppm) 
Other features, advantages and embodiments of the invention disclosed 
herein will be readily apparent to those exercising ordinary skill after 
reading the foregoing disclosures. In this regard, while specific 
embodiments of the invention have been described in considerable detail, 
variations and modifications of these embodiments can be effected without 
departing from the spirit and scope of the invention as described and 
claimed.