WO1996017017A1

WO1996017017A1 - Polystyrene having improved organoleptic properties

Info

Publication number: WO1996017017A1
Application number: PCT/EP1995/004539
Authority: WO
Inventors: Dietmar Bender; Rainer Moors; Hans-Dieter Schwaben
Original assignee: Basf Aktiengesellschaft
Priority date: 1994-11-29
Filing date: 1995-11-18
Publication date: 1996-06-06
Also published as: DE4442447A1

Abstract

The invention concerns thermoplastic shaping compounds comprising a mixture of anionically polymerized general purpose polystyrene and high-impact polystyrene which is low in monomers, preferably with a weight/mixture ratio of 10: 1 to 1: 10. Each component of the mixture contains fewer than 100 ppm of monomer styrene and, for example, the anionically polymerized general purpose polystyrene has a degree of molecular heterogeneity Mw/Mn of more than 1.5. High-impact polystyrene with an average particle diameter of more than 0.3 νm is used. The invention further concerns thermoformed packaging for foodstuffs, said packaging being produced using this thermoplastic shaping compound.

Description

Polystyrene with improved organoleptic properties

description

Polystyrene, and in particular mixtures of so-called standard polystyrene (GPPS - General Purpose Polystyrene) and impact-modified polystyrene (HIPS - High Impact Polystyrene) are widely used for food packaging. By using a mixture of GPPS and HIPS, you combine the rigidity of the GPPS with the toughness of the HIPS and you get materials that are particularly suitable for this application. A disadvantage of the mixtures of GPPS and HIPS with regard to the so-called organoleptic properties (neutral odor or taste) is the manufacturing-related residual styrene monomer content, which when using conventional methods for demonomerization (so-called "degassing" ) is in the range around 500 ppm. Even when using the latest degassing technologies, a limit of 100 ppm residual styrene in GPPS can practically not be fallen below. In the case of organoleptically sensitive filling goods such as cookies, dairy products or chocolate, styrene causes changes in the taste.

Almost monomer-free polystyrene (below 5 ppm) with excellent organoleptic properties can be produced by anionic polymerization using organometallic compounds of lithium, such as n-butyllithium in non-polar solvents (see M. Swarc: "Living Polymers and Mechanisms of Anionic Polymerization" in Adv. Polym. Sci. 49, Heidelberg 1983).

In the water-free and oxygen-free medium, the (so-called living) anionic polymerization, depending on the process design, leads to polystyrene (A-GPPS) with either narrower (M _w / M _n = 1.05-1.2) or with broader, radically produced polystyrene Molar mass distribution (M _w / M _n = 1.9 - 2.5) - cf. J. Appl. Polym. Be. 37: 393-402 (1989); J. Appl. Polym. Be. 37, 1079-1088 (1989) and EP-A-0176 611.

The use of narrowly distributed A-GPPS to improve the processing properties of HIPS is described in EP-A-460497.

Processes for the production of impact modified polystyrene by anionic route with the mechanical properties of radically produced HIPS are currently not known or at least not implemented on an industrial scale. The organoleptic advantages of the A-GPPS can therefore only be used for stiff / brittle applications. The aim of the invention is to produce impact-resistant polystyrene molding compositions for packaging purposes with markedly improved organoleptic properties and reduced styrene migration.

According to the invention, the improvement in organoleptic properties is achieved by mixing anionically polymerized polystyrene (A-GPPS) with radical-produced impact-resistant polystyrene, the monostyrene content of which has been reduced to below 100 ppm by the addition of an entrainer in the degassing the mixtures are processed in the usual manner, for example to give films and packaging. Broadly distributed A-GPPS (M _w / M _n > 1.5) is preferably used, which has significantly better mechanical properties than anionic polystyrene with a narrow molar mass distribution (cf. Table 2). Through the use of particularly low-monomer combinations, such as the combination of A-GPPS with a residual styrene content of less than 10 ppm and HIPS with a residual styrene content of less than 50 ppm, finished parts with drastically reduced styrene migration are accessible (see Fig. l).

A further parameter for A-GPPS preferred according to the invention is a number-average molecular weight (M _n ), which is preferably between 50,000 g / mol to 500,000 g / mol. A-GPPS, whose melt volume rate (MVR 200/5) according to ISO 1133 is in the range from 0.5 to 30 ml / 10 min, is very particularly preferred.

In principle, all known, either described or commercially available varieties can be used to obtain low-monomer HIPS. The particle structure is only of secondary importance for the migration properties. Due to the good mechanical properties of the resulting mixtures with A-GPPS, preference is given to a HIPS with rubber particles whose average particle diameter is greater than 0.3 μm and which generally have capsule or cell particle morphology. Products with the addition of a so-called internal lubricant, e.g. of an aliphatic carbon hydrogen, usually have a higher styrene migration than unlubricated products with the same styrene content.

Mixtures of an A-GPPS with less than 10 ppm styrene and a HIPS with less than 50 ppm are particularly suitable as mixing partners for the production of containers for food (cups, bowls, trays). The mixing ratio of the two components is based on the requirement profile of the respective application and is usually in the range from 10: 1 to 1:10. Mixtures in the range HIPS / A-GPPS 4: 1 to 1: 2 are preferred. A high proportion of A-GPPS results in a reduction in the specific migration of styrene with an identical monostyrene content (see FIG. 1). The sensory advantages of the beverage cups produced from the A-GPPS / HIPS mixtures according to the invention compared to those from conventional GPPS / HIPS mixtures (styrene content 350 ppm or more) can be determined in accordance with DIN 10 963 (cf. Table 1).

The mixing of A-GPPS and low-monomer HIPS makes thermoplastic molding compositions accessible which have excellent organoleptic properties and extremely low migration of styrene.

When processing low-monomer A-GPPS and HIPS, make sure that the melt temperature during extrusion does not exceed 250 ° C. Otherwise monomer cleavage occurs. For the same reason, extruder zones should be avoided (blind spots), in which the material remains unnecessarily long.

The following products were provided to demonstrate the effect according to the invention in the examples below:

Anionic polystyrene (A-GPPS)

The anionic polystyrenes used were prepared in a continuously operated stirred kettle in ethylbenzene as the solvent. The inconsistency M / n of the products was between 1.9 and 2.6. Solvent and residual monomer from A-GPPS was removed by means of a degassing extruder and strand degassing with the addition of entrainer (water).

Low-monomer impact polystyrene (HIPS)

Low monomer impact polystyrene was obtained according to the method described by A. Echte, Advances in Chemistry, Series 222 (Rubber-Toughened Styrene Polymers, Fig. 27, p 39), the styrene content being able to be reduced to below 40 ppm by using an entrainer. The impact-resistant polystyrene products used had a polybutadiene content of 8% with an average particle diameter of 2.7 μm and corresponded to the BASF range products 476L and 486M in other properties.

Processing into cups and test specimens

The mixtures were produced from granules of the respective components in an extruder and processed by wide-slot film extrusion at a melt temperature of 220 ° C to 1.4 mm thick films, which at 140 ° C to standardized 200 ml Beverage cups were thermoformed. The styrene content was checked in all processing stages.

Test specimens for migration measurements were injection molded at 220 ° C. The styrene content was determined before and after processing.

Migration measurements:

Experience has shown that one of the most critical test media for polystyrene is olive oil, in which high migration values are generally found. For the investigations, standard small sticks were stored in olive oil at 40 ° C for 10 days. A comparison of the migration values of HIPS / PS blends with the HIPS / A-GPPS blends shows the advantages of the A-GPPS. In a first approximation, there is a linear relationship between the residual styrene content in the polystyrene molding composition and the value of the specific styrene migration (Table 3 and Fig. 1).

Specific styrene migration

The migration measurements were carried out with standard olive oil on standard small sticks, each of which was stored for 10 days at 40 ° C. The relevant amount of styrene was determined using steam roughness -GC / MS with n-butylbenzene as the internal standard.

Commercially available products from BASF Aktiengesellschaft, namely polystyrene 476L (39 ppm residual styrene, entrainer degassed), polystyrene 486 Mma (100 ppm residual styrene), polystyrene KR 2601ma (100 ppm residual styrene, both entrainer degassed), as well as the range products 486M and 165 H with 350 ppm residual styrene and finally A-GPPS (4 ppm residual styrene) examined - cf. Table 1.

Table 1

Styrene migration measurements on injection molded standard small bars

ma = low monomer content

* Measurements on the granulate

** Measurements on injection molded standard small bars

Sensory tests

For the purpose of sensory assessment, thermoformed 200 ml beverage cups were filled with boiling distilled water and an olfactory assessment was made immediately after filling. The four samples tested were tested in accordance with DIN 10963. The best rated material was ranked 1, the second best rank 2, etc. The result is shown in Table 2 below.

Sample 1 beverage cup made from A-GPPS / HIPS in a mixing ratio of 50/50 with 4 ppm or 39 ppm residual styrene

Sample 2 beverage cups made from PS KR 2601ma / HIPS 486Mma in a mixing ratio of 50/50 with 100 ppm or 100 ppm

Residual styrene

Sample 3 beverage cups made from PS KR 2601ma / HIPS 486M in a mixing ratio of 50/50 with 100 ppm or 350 ppm residual styrene

Sample 4 beverage cups made of PS 165H / HIPS 486M in a 50/50 mixture ratio with 300 ppm / 350 ppm residual styrene. Table 2

Sensory tests according to DIN 10963

Inspector Sample 1 Sample 2 Sample 3 Sample 4

1 1 3 2 4

2 1 2 3 4

3 1 2 4 3

4 1 4 2 3

5 3 2 4 1

6 2 1 3 4

7 1 2 3 4

8 2 1 4 3

9 1 2 3 4

10 1 3 4 2

11 3 2 1 4

12 1 3 2 4

Rank 18 27 35 40

Sample 1 according to the invention achieves a significance α of 0.01

Table 3

Comparison between widely distributed and narrowly distributed A-GPPS

Material A-GPPS 1 A-GPPS 2

M _n / M _M 2.5 1.1

MVR 200/5 [cm / 10 min.] 5.7 4.6

VST B / 50 [° C] 102 102.3

VST A / 50 PC] - -

VZ [ml / g] - -

Yield stress [MPA] 0 0

Tensile strength

[MPA] 50 45

Elongation at break [%] 2 1.5

Modulus of elasticity [MPA] 3200 3000

Impact bending verse. 179 / leA + 23 ° C 17 12 [KJ / m.2] notched

Impact bending verse.

179 / leU + 23 ° C 3 2

[KJ / m.2]

V-notch r = 0.25 mm

Impact strength after Izod 180 / 1C + 23 ° C 10 7 [KJ / m2]

Notched impact strength 2 1.5 after Izod 180 / 1A + 23 ° C [kJ / m2]

Claims

claims

1. Thermoplastic molding composition, namely a mixture of anionically polymerized standard polystyrene (A-GPPS) and low-monomer impact-modified polystyrene (HIPS), characterized in that both mixture partners each contain less than 100 ppm of monomeric styrene.

2. Molding composition according to claim 1, containing A-GPPS and HIPS in a weight mixing ratio of 10 to 1 to 1 to 10.

3. Molding composition according to claim 1 and 2, characterized in that the A-GPPS used has a ratio M _w / M _n over 1.5.

4. Molding composition according to claims 1-3, characterized in that impact-modified polystyrene with an average particle diameter above 0.3 µm is used.

5. Thermoformed food packaging made from a thermoplastic molding composition according to one of claims 1 to 4.

6. Thermoformed food packaging according to claim 5, characterized in that the styrene migration measured in olive oil at 0 ° C and 10 days storage time is below 20 ppb.

Polystyrene with improved organoleptic properties

Summary

Thermoplastic molding composition made from a mixture of anionically polymerized standard polystyrene and low-monomer impact-modified polystyrene, preferably in a weight mixing ratio of 10 to 1 to 1 to 10, where each mixing partner contains less than 100 ppm of monomeric styrene and, for example, that ¬ ionically polymerized standard polystyrene has a molecular non-uniformity M _w / M _n over 1.5 and impact-modified polystyrene with an average particle diameter over 0.3 m is used, as well as thermoformed food packaging made from this thermoplastic molding compound.