Process for Injection Molding Polyolefin Resin Compositions

This invention relates to a polyolefin resin composition suitable for obtaining injection molding products with good transparency.

It is, hitherto, known that an improvement of a polyolefin injection molding product in transparency is attained with use of additives such as dialkyleneglycol alkyl ethers, benzenesulfonic acid esters, ammonium salt of benzenecarboxylic acid, thiodicarboxylic acids, aluminum salt of di-p-t-butylbenzoic acid, dibenzylidene-D-sorbitol or the like. For example, condensates of sorbitol with aldehyde compounds such as dibenzylidenesorbitol or 1.3,2.4-di(methylbenzylidene)sorbitol are disclosed in Japanese Tokkyo Kokai No. 51-22740 and No. 53-117044. These condensates are useful in improving the transparency, but there are disadvantages that foams or blisters often occur on the surface of or inside molding products so that the practical value is lost.

An object of this invention is to provide polyolefin resin compositions suitable for producing injection molding products having good transparency without presenting a foaming or blistering phenomenon.

In accordance with this invention, there is provided a polyolefin resin composition for injection molding which comprises adding to polyolefin a condensate of sorbitol with an aldehyde compound and a lubricant of 1/20-1/2 parts by weight based on 1.0 part by weight of said condensate. Injection molding products of polyolefin having good transparency without bringing a foaming phenomenon are obtained by using a condensate of sorbitol with an aldehyde compound in combination with a lubricant.

The polyolefin which may be used in this invention includes propylene homopolymer, propylene-ethylene random copolymer, propylene-ethylene block copolymer, propylene-butene-1 random copolymer, low density ethylene polymers, ethylene-propylene random copolymer, ethylene-butene-1 random copolymer, butene-1-ethylene random copolymer, butene-1-propylene random copolymer and the like. These may be used alone or in mixture of two or more.

The sorbitol-aldehyde condensate which may be used in this invention is represented by the formulae, ##STR1## wherein R.sub.1 and R.sub.2 are each a saturated or unsaturated alkyl group having 1-30 carbon atoms, for example, ethyl, n-hexyl, iso-hexyl, n-undecyl, 2-nonyl-2-butenyl and others;

an alicyclic hydrocarbon group or its derivatives, for example, cyclohexyl, p-methyl-cyclohexyl and others; and

an aromatic hydrocarbon group or its derivatives, for example, phenyl, p-methylphenyl, m-methylphenyl, o-methylphenyl, p-ethylphenyl, p-hexylphenyl, 1-naphthyl, 1-anthryl, benzyl and so on.

The following are typical examples of these condensates which may be used alone or in mixture of two or more.

R.sub.1 and R.sub.2 ; Saturated or unsaturated alkyl groups of C.sub.1 -C.sub.30 :

1.3-heptanylidenesorbitol,

1.3,2.4-diheptanylidenesorbitol,

1.3,2.4-di(3-nonyl-3-pentenylidene)sorbitol,

Alicyclic hydrocarbon groups or derivatives thereof:

1.3-cyclohexanecarbylidenesorbitol,

1.3,2.4-dicyclohexanecarbylidenesorbitol,

1.3,2.4-di(p-methylcyclohexanecarbylidene)sorbitol,

Aromatic hydrocarbon groups or derivatives thereof:

1.3-benzylidenesorbitol,

1.3,2.4-dibenzylidene-D-sorbitol,

1.3,2.4-di(m-methylbenzylidene)sorbitol,

1.3,2.4-di(p-ethylbenzylidene)sorbitol,

1.3,2.4-di(p-hexylbenzylidene)sorbitol,

1.3,2.4-di(1-naphthalenecarbylidene)sorbitol,

1.3,2.4-di(phenylacetylidene)sorbitol.

A process for the synthesis of the sorbitol-aldehyde condensates is not particularly limited, though in general they are obtained by heating a mixture of sorbitol and an aldehyde compound in the presence of a dehydration catalyst such as anhydrous phosphoric acid or sulfuric acid. Such a synthesis process is disclosed in, for example, Japanese Patent Publication No. 47-7460. In the case of dibenzylidenesorbitol, 45.9 g of sorbitol, 39.8 g of benzaldehyde and 4 g of a concentrated sulfuric acid are charged to a container provided with a reflux condenser, a Liebig condenser, a liquid pouring tube (safety tube), a stirrer and a gas inlet, heated on an oil bath to 178.degree. C. and maintained at same temperature for 30 hours. Then, after cooling, the temperature is lowered to about 60.degree. C., 50 cc of methanol are added from the safety tube and the temperature is elevated to 100.degree. C. At that time a mixture of benzaldehyde, methanol and water is effected. After completion of the effluence the reaction product mixture is cooled, poured into a large quantity of water to remove sulfuric acid and then, benzaldehyde is washed out with hot water. Thus a white solid dibenzylidenesorbitol is obtained. Other condensates may be obtained in the same procedure. Also, the starting sorbitol may be the D-form, L-form or racemic-form.

The sorbitol-aldehyde condensate may be used in the amount of preferably, 0.05-2.0 parts, more preferably 0.1-1.0 parts by weight based on 100 parts by weight of polyolefin. With an amount of less than 0.05 parts, the effect of improving transparency is hardly attained, whereas with an amount of more than 2.0 parts the effect of improving transparency is reduced on the contrary and also the product obtained becomes expensive.

The lubricants which may be used are, for example, saturated or unsaturated aliphatic amides and derivatives thereof, e.g. hexic acid amide, octic acid amide, stearic acid amide, oleic acid amide, erucic acid amide, ethylenebisstearic acid amide, lauric acid amide, behenic acid amide, methylenebisstearic acid amide, ricinoleic acid amide and so on: saturated or unsaturated aliphatic esters having 3-30 carbon atoms and derivatives thereof, e.g. butyl stearate, isobutyl stearate and so on: silicone compounds, e.g. a commercially available silicone mold release agent, silicone oil and so on. The silicone compounds are, preferably, linear organopolysiloxanes having a viscosity of 10-100,000 centistokes and more preferably, a straight chain dimethylpolysiloxane.

These lubricants may be used alone or in mixture of two or more within the range of preferably, 1/20 to 1/2 parts and more preferably, 1/10 to 1/4 parts based on 1.0 parts by weight of the sorbitol-aldehyde condensate. When the amount is less than 1/20 parts, a foam controlling effect is not attained. On the other hand, when the amount exceeds 1/2 parts, the foaming or blistering is prevented, but there are disadvantages that the effect of improvement in transparency is reduced, molds are remarkably contaminated and the cost for production of moldings becomes expensive.

The composition of this invention may be used in combination with antioxidants, dyestuffs, ultraviolet absorbers and others. This invention will be illustrated by the following Examples.

Indicated amounts of the sorbitol-aldehyde condensate, indicated amounts of erucic acid amide and 0.1 part by weight of 2,6-di-t-butyl-4-methylphenol were mixed with 100 parts by weight of a propylene-ethylene random copolymer [ethylene content: 3.5% by weight, melt flow index: 15 g/10 minutes (ASTM-D-1238-62T)], which were then extruded with a 65 mm .phi. extruder at 250.degree. C. to obtain pellets.

The pellets obtained were formed to an injection molding sheet of 7.8 cm width.times.15.7 cm length.times.2 mm thickness with an injection molding machine at 250.degree. C. and then, tested in respect of transparency and a degree of foaming. The transparency is indicated by a haze (%) (or a degree of cloudiness) measured in accordance with JIS-K 6714. The results are set forth in Table 1, which show that the process of this invention is extremely effective for the prevention of foaming.

Injection molding sheets were obtained in the same procedure as in Example 1 except using a libricant set forth in Table 2 instead of erucic acid amide. The test results are given in Table 2.

It is clear therefrom that the process of this invention is remarkably effective for the prevention of foaming.

Industrial Applicability:

As described above, when the compositions of this invention are subject to injection molding, the transparency of the moldings is remarkably improved without presenting a foaming phenomenon and thus, transparent moldings having a good appearance are obtained.