GB2116987A - Polypropylene-containing mixtures for use in making construction units - Google Patents

Abstract

A composition suitable for making crosslinked and optionally foamed shaped structures comprises polypropylene with 2 to 20% of its weight of polybuta-1,2-diene having a molecular weight of 500 to 10,000 and with 10 to 50% of its weight of high- density or low-density polyethylene and optionally also a cross-linking agent and/or a foaming agent. After shaping the composition and subjecting it to a dose of 0.5 to 20 Mrad of high-energy radiation and/or a heating operation in an oven for the purposes of crosslinking and, if appropriate, foaming, the resulting materials can be used as construction units.

Description

SPECIFICATION Polypropylene-containing mixtures for use as construction units The invention relates to a process for the manufacture of a crosslinked and optionally foamed mixture containing polypropylene, in which polypropylene is mixed with 2-20% of its weight of a polybuta-1,2-diene having a molecular weight of 500 to 10,000 and optionally also with a crosslinking agent and/or a foaming agent, a temperature below the decomposition temperature of the crosslinking agent or the foaming agent bing used in that event, the mixture is shaped and the shaped mixture is subjected to a dose of 0.5 to 20 M rad, preferably 2 to 20 Mrad, of high-energy radiation and/or a heating operation in an oven for the purposes of crosslinking and, if appropriate, foaming.

The process of this type is described in German Patent Specification 2,839,733. It leads to products which, from the point of view of thermal properties, have both the advantages and the disadvantages of polypropylene. As regards disadvantages, the strong tendency towards embrittlement at low temperatures is particularly unsatisfactory.

Japanese Published Patent Application 49,825/ 1972 discloses a process for foaming polyolefins containing 0.3 to 40 parts by weight of a rubbery polybuta-1 ,4-diene, in a direct gasification process, that is to say by a physical method. The polybuta-1 4-diene content leads to the formation of small and stable cells, but has the effect of reducing the heat stability with increasing temperatures and hence of impairing the heat resistance of the foam obtained.

German Offenlegungsschrift 2,937,528 discloses a foam based on a mixture of polypropylene, polybuta-1 ,2-diene, other thermoplastic resins, for example an ethylene/a-olefin copolymer, and also a propellant. The foam is distinguished by an especially high heat resistance and a uniform cell structure and also by a pleasing appearance. However, vacuum forming is only possible to a limited extent, and low temperatures in the region of the freezing point lead to substantial embrittlement.

German Auslegeschrift 1,694,130 refers to a process for the manufacture of a foam, in which a polyolefin, a polybuta-1,4-diene rubber and also an organic peroxide are mixed, and the mixture is shaped and heated for subsequent crosslinking and foaming. These foams are very soft and flexible. The low rigidity excludes use for the manufacture of self-supporting facing elements.

The present invention seeks to provide a process for the manufacture of a crosslinkable and optionally also foamable mixture containing polypropylene, which makes it possible to manufacture high-grade construction units. Together with a good heat resistance, corresponding to the properties of pure polypropylene in the region of the freezing point, these construction units should be ideally distinguished by the complete absence of the known embrittlement of polypropylene and also by a good viscosity and rigidity in the temperature range from -20 to +130"C. Plate-shaped semi-finished products made from the said mixture should be convertible by application of the vacuum technique.The extremely fine cell structure and smooth surface in foamed embodiments should in no way be inferior to those of other known high-grade foams.

According to the invention there is provided a process of the type defined at the outset in which 10 to 50% by weight, relative to the weight of the polypropylene, of high-density or low-density polyethylene is added to the mixture used.

The polymeric mixture used in the process therefore consists predominantly of polyethylene and polypropylene with a smaller proportion of liquid polybuta-1,2-diene.

Surprisingly, this mixture has new properties different from the specific properties of the individual polymers used. The new properties are not even in a boundary region closes to the properties of the individual polymers and to this extent were not predictable.

The proposed process makes it possible to manufacture construction units having a satisfactory heat distortion point. To determine the heat distortion point of the foam produced in Example 1 below with a density of 30 kg/m3, a 10 mm thick plate was cut out of this foam and, using a commercially available vacuum forming machine, was shaped into a cylindrical cap with an external diameter of 120 mm and a height of 60 mm. This cap was stored for 24 hours at 150"C in a circulating air oven, as a result of which it suffered a 3.3% decrease in its diameter and also a 4.8% decrease in its height. Comparable results were obtained using a pure polypropylene foam of the same density but lacking polyethylene.

In another experiment, the said plate-shaped foam obtained according to Example 1 was stored for a period of 24 hours at a temperature of 150"C in a circulating air oven. The shrinkage was about 1% in the longitudinal direction and transverse direction.

The comparable pure polypropylene foam also showed a shrinkage of about 1% in the longitudinal direction and transverse direction. The temperature was then raised to 1600C in both cases. Both the pure polypropylene foam and the foam produced according to the invention shrank substantially. It can be inferred from this that the heat distortion point of the foam produced according to the invention is more or less the same as that of the pure polypropylene, despite a considerable content of high-density polyethylene which has a softening point of 1300C.

Samples of the foam produced according to Example 1 were then subjected to a torsional oscillation test according to DIN 53,445. In this test, it was shown that a maximum in the damping decrement appears at 4.1 Hz = Tt = -40 C. Neither the pure polypropylene foam nor the pure high-density polyethylene has this damping maximum. Figures 1, 2 and 3 respectively of the accompanying drawings show the change in damping decrement with temperature for the foam produced according to Example 1, pure polypropylene foam and pure high-density polyethylene.

Such maxima ofthe damping decrement give information on the behaviour of materials at low temperatures. The foam produced according to Example 1 has a transition temperature of -40 C according to Figure 1, that is to say it only embrittles below this temperature; in contrast, pure polypropylene foam already embrittles at a temperature of about +7"C. In the region of the freezing point, the material produced according to the invention is to this extent superiorto the pure polypropylenefoam.

Figures 4 and 5 of the accompanying drawings show the damping behaviour of the various materials as a function of the temperature. Curve A in Figure 4 represents the damping behaviour of a foam substance made from the mixture according to Example 1, and curve B in Figure 4 represents the damping behaviour of a comparison substance made of pure polypropylene and having identical dimensions and the same density. Both curves are of almost identical shape. A sharp drop appears only above a temperature of 1 50"C.

In the case of the comparison substance made of non-foamed high-density polyethylene, a similar drop already occurs at a temperature of 130 C (Figure 5).

When using high-density polyethylene as an additive to the mixture in accordance with the invention, the required amount of crosslinking agent (peroxide content or radiation dose) can be reduced by 10 to 20%, compared with the addition of low-density polyethylene. Hence, the choice is of considerable benefit also from the economic point of view.

The process according to the invention is also suitableforthe manufacture of crosslinked nonfoamed polyolefin materials. These can preferably be further processed to give supporting construction units. In the temperature range between -40 and +1500C, they are distinguished by properties which appear to be markedly superior to those of the known polyolefin materials.

The present invention is further illustrated in the following Examples.

Example 1 66.3 parts by weight of high-density polypropylene (melt index 230/5 g/10 min. = 10), 18.7 parts by weight of high-density polyethylene (melt index 190/2.16 g/10 min. = 4.6; (p = 0.927 - 0.929 g/cm3) and 15 parts by weight of azodicarbonamide are compounded in a twin-screw extruder with 5.6 parts by weight of polybutadiene (molecular weight: 3000, polybuta-1,2-diene content: 90% by weight). The temperature of the mass during compounding is adjusted to 175-185"C. The mixture homogenised in the extruder is shaped into a web through a wide-slit nozzle and then crosslinked by means of electron radiation with a surface dose of 7 Mrad.

The crosslinked product thus obtained is conveyed onto a travelling screen through a circulating air oven and thereby foamed. The dwell time in the oven is 8 minutes at a temperature of 215 C. The foam plate has a uniform cell structure and a density of 30 kg/m3.

The proportion of gel in the material is determined in boiling xylene as follows: 1 g of material is boiled for 5 minutes in 100 ml of xylene, filtered off and rinsed with 3 x 50 ml of hot xylene. After the insoluble filtrate has been dried for 16 hours at 1050C, the proportion of gel in the material used is determined by weighing the insoluble filtrate.

The proportion of gel is 67%.

The test results for the foam are shown in Figures 1 and 4; in the latter graph, the reference curve is curve A.

Example 2 85 parts by weight of high-density polypropylene (melt index 230/5 g/10 min.= 10) and 15 parts by weight of azodicarbonamide are compounded in a twin-screw extruder with 5.6 parts by weight of polybutadiene (molecular wieght: 3000, polybuta-1 ,2-diene content: 90% by weight). The temperature of the mass during compounding is adjusted to 175-185"C. The mixture homogenised in the extruder is shaped into a web through a wide-slit nozzle and crosslinked by means of electron radiation with a surface dose of 8 Mrad.

The crosslinked product is conveyed onto a travelling screen through a circulating air oven and thereby foamed. The dwell time is 8 minutes at a temperature of 215 C. The foam web obtained has a uniform cell structure and a density of 30 kg/m3.

The test results for this foam are shown in Figures 2 and 4; in the latter diagram, the reference curve is curve B.

The differences between the two products of Example 1 and Example 2 are clearly apparent. The foam produced by the process according to the invention has a transition point at -40 C, which occurs neither in the case of pure polypropylene (Figure 1) nor in the case of pure high-density polyethylene (Figure 3).

This result is confirmed in the following Examples.

Example 3 74.3 parts by weight of polypropylene (melt index 230/5 g/10 min. = 10), 20.7 parts by weight of high-density polyethylene (melt index 190/2.16 g/10 min. = 4.6; q = 0.927 - 0.929 g/cm3) and 5 parts by weight of azodicarbonamide are compounded in a twin-shaft extruder with 5.3% by weight of polybutadiene (molecular weight: 3000, polybuta-1,2-diene content: 90% by weight) and processed further as in Example 1.

The foam obtained has a density of 75 kg/m3. The proportion of gel is 69%.

The logarithmic damping decrement according to DIN 53,445 is shown in Figure 6 of the accompanying drawings.

Example 4 95 parts by weight of polypropylene (melt index 230/5g/10 min. = 10) and 5 parts by weight of azodicarbonamide are compounded in a twin-shaft extruder with 5.3% by weight of polybutadiene (molecular weight: 3000; polybuta-1,2-diene content: 90% by weight) and processed further as in Example 2.

The foam obtained has a density of 80 kg/m3. The proportion of gel is 70%.

The logarithmic damping decrement according to DIN 53,445 is shown in Figure 7 of the accompanying drawings.

Claims (7)

1. A process for the manufacture of a crosslinked shaped mixture containing polypropylene, in which polypropylene is mixed with 2 to 20% of its weight of polybuta-1,2-diene having a molecular weight of 500 to 10,000 and with 10 to 50% of its weight of polyethylene the mixture is shaped and the shaped mixture is subjected to a dose of 0.5 to 20 Mrad of high-energy radiation and/or a heating operation in an oven for the purposes of crosslinking.

2. A process as claimed in claim 1, wherein the polyethylene used is high-density polyethylene.

3. A process as claimed in claim 1 or 2, wherein the mixture also contains a foaming agent prior to shaping and a foam is produced as a result of the irradiation and/or heating.

4 A process for the manufacture of a crosslinked shaped mixture containing polypropylene carried out substantially as described in the foregoing Examples orExample3.

5. A construction unit manufactured by a process as claimed in any of claims 1 to 4.

6. A ptityrner rnixture comprising 100 parts by weight of polypropylene, 10 to imparts by weight of polyethylene, 2 to 200 parts lwweight of liquid polybutadiene, and optionally a crosslinking agent and/or a foaming agent.

7. A polymer mixture as claimed in claim 6 wherein high density polypropylene and high-density polyethylene are used.