K. FIGGE and W. FREYTAG
Unilever Forschungsgesellschaft mbH, Behringstraße 154, Postfach 50 15 68. D-2000 Hamburg 50, FR Germany
(Received 15 October 1984; accepted 25 October 1984)
Each plastic was processed into test specimens such as pressed and extruded sheets, injection-moulded cups, deep-drawn tubs and blown bottles. The migration out of these specimens was investigated under identical test conditions. The results confirm that the amounts of additive migrating from the different classes of plastics into test fat HB 307 in general decrease in the order LDPE > HDPE > PP > HIPS. Moreover, it seems to be of great importance that the respective amounts of additive migrating from the injection-moulded cups, deep-drawn tubs and blown bottles into test fat were significantly lower in all cases than those from the corresponding pressed or extruded sheets. Presumably, this effect is mainly caused by orientation of the polymer molecules in the injection-moulded, deep-drawn or blown products.
The migration of plastics packaging components into foodstuffs is essentially a problem of diffusion. The extent of migration of a particular plastics component from a food packaging material into the contained foodstuff depends on a number of variable factors (Figge 1983): viz. concentration of the component in the plastics material, the temperature in the contact system of plastic/foodstuff, and the duration of contact between the system components. Generally, if t (the contact time) and T (the temperature of the contact system) are constant:
where M is the quantity of migrated component and C0 its initial concentration in the plastic. If C0 and t are constant, M and T are related by the proportionality:
(Figge and Klahn 1982, Klahn and Figge 1982, Figge 1980).
On the other hand, the fraction of a component migrating under practical or test conditions from a plastics packaging material into a test food depends very significantly on the physicochemical characteristics of the respective contact phases, e.g. density of the plastics packaging material, orientation degree of macromolecules in its surface, polarity and viscosity of the contact media. Regardless of these findings, in the course of drafting specifications for migration tests only the relationship between the fat release* of the packed products and their capacity to promote migration of plastics components have been the subject of systematic investigation (Bieber et al. 1983, Koch and Figge 1978, Koch et al. 1976). There is no doubt that all these parameters should have been studied, as they are essential experimental factors.
* The amount of fat transferred from a food to an LD-PE-sheet under defined conditions.
(a) In order to estimate-for filing an indirect food additive petition to authorities, for example BEA and FDA-the migration of components, such as processing additives, from plastics into packed products, especially foodstuffs, test sheets are usually prepared from the plastics materials under investigation. These test sheets are brought into contact with the test food under defined test conditions and the migration of the additive is evaluated by conventional or radioanalytical methods. However, not all foodstuffs are packed in plastic sheets, the majority being packed in containers such as cups, tubs and bottles. The use of test sheets is based on the assumption that the amounts of components migrating from containers prepared from any plastics material are not greater than those migrating from a pressed or extruded sheet prepared from the identical plastics blend. This assumption is supported by gas diffusion measurements (Herrero 1974), but has not yet been proved by migration data. Thus the initial aim of our work was to investigate the influence of the processing of plastics (pressing, extruding, injection moulding, deep-drawing, blowing) on the behaviour of plastics packaging materials with regard to the migration of their mobile, i.e. low molecular, diffusing components into fat.
Accordingly, in the second part of the report, migration data will be presented which have been obtained in test series using various types of plastics belonging to the same class of plastics. The test conditions were identical in all cases.
The influence of processing
Experimental procedure
The selected plastics materials used were: Lupolen 5661 BX, density 0.952-0. 956 g/cm3, a high-density polyethylene (HDPE), Novolen 1120 HX, density 0.907 g/cm3, a polypropylene (PP) and SB 466 I, impact strength 60 kJ/m2, a high-impact polystyrene (HIPS) (all from BASF, Ludwigshafen, FR Germany). The plastics were ground and 99.9% by weight of each was mixed thoroughly with 0.1 % by weight of n-octadecyl 3-(3,5-di-tert-butyl-4hydroxyphenyl)-[3-14C] propionate, a 14C-labelled representative phenolic antioxidant, hereafter called `the additive'. One half of each basic blend thus produced was directly pressed to test sheets; the other was first extruded and granulated and subsequently processed under technical processing conditions into the following plastics products (figure I):
Circular test specimens (60 mm diameter) were prepared from the HDPE and PP sheets and brought into one-sided contact with test fat HB 307 (Figge et al. 1972) using migration cells. The sheets produced from HIPS were cut into rectangular pieces (100 mm x 14 mm). They were brought into contact on all sides with the test fat in glass vials; they interact with the test fat under the test conditions only in a thin surface layer.
The cups, tubs and bottles were filled with test fat HB 307 so that one-sided contact was achieved. The cups were 7 cm high and had an upper diameter of 5.5 cm and a lower diameter of 4.5 cm; the tubs had flat bottoms, a height of 3 cm and a diameter of about 6 cm; the cylindrical bottles had an external diameter of about 8 cm and a total height of about 17 cm.
The test conditions were 10 days at 40°C. We measured the transfer of the additive from the different test materials into test fat HB 307 by radioanalytical methods (Figge 1980, Figge 1978, Figge 1976, Figge and Piater 1971).
ResultsThe migration trials led to the results shown in table 1. It can be seen that, independent of the type of test specimen used in the trial, the maximum amount of additive always migrated into the test fat from the HDPE test specimens. In corresponding tests, the smallest quantities of additive migrated from the HIPS test specimens. This finding confirms that the migration of a particular substance from different classes of plastics into identical contact media can differ widely.
One new fact has now been verified: the quantity of additive which migrates into test fat HB 307, under identical test conditions, from HDPE, PP and HIPS depends on the processing of the test specimen. Only in the case of two plastics sheets prepared from one blend via different processing routes were the quantities of additive migrating into the test fat nearly the same. The corresponding migration values using injection-moulded cups, deep-drawn tubs and blown bottles as test specimens were much lower and also different from one another. Thus, for PP, about 580µg additive per dm' contact area migrated into the test fat HB 307 from the extruded sheets, but only 1 13 µg additive per dm' migrated from injection-moulded cups (about 19 5% of the quantity released by sheets). Similarly, for HIPS, injection-moulded cups release under the standard test conditions 10 days/40°C only about 1 1 5% of the quantity of additive released by extruded sheets.
Figure 1. Preparation of the different test specimens from one type of plastic
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Migrated amount of additive a
(µg/dm2)
(mean and SD, n = 10)
Type of specimen
(thickness approx. 800µm) HDPE PP HIPS
Pressed sheet from basic blend 9700 652.3 20.4
(43.2) (83.0) (1.7)
Pressed sheet from granulate 9020 692.3 18.5
(11.7) (122) (0.47)
Extruded sheet from granulate 9549 582.2 22.6
(11.8) (161) (0.51)
Injection-moulded cups 466.8 113.9 2.61
from granulate (19.4) (0.56) (0.08)
Deep-drawn tubs from 794.2 497.7 1.31
extruded sheets (4.8) (9.1) (0.06)
Blown bottles from extruded 525.6 364.9 0.97
tube of granulates (4.4) (4.3) (0.1)
a Related to an additive concentration in the test specimen of 0 1% by weight.
The results of the investigations can be summarized as follows. Nearly equal quantities of the additive migrated from the test sheets prepared from one class of plastics by two different methods. However, the respective amounts of additive migrating from the injection moulded HDPE, PP and HIPS cups were 51-89%, from the deep-drawn tubs 14-95% and from the blown bottles 37-96% lower than those transferred from extruded sheets into the test fat. These findings are of direct significance for test practice.
Initial investigations regarding the structural nature of the superficial layers of the different readymade plastics specimens showed that, compared to the two sheets prepared by different methods, specimens prepared by injection moulding, deep-drawing and blowing have macromolecules on their surface having a significantly higher degree of orientation. Therefore, it may be that such barriers of higher-oriented macromolecules prevent both penetration of fat into the plastics containers and migration of mobile components from the plastics containers into fat-releasing test media.
The influence of differing properties of plastics within one classExperimental procedure
We selected four plastics types having different properties-different densities, melt flow indices and structural characteristics-from each of the plastics classes LDPE, HDPE, PP and HIPS, as shown in table 2.
Melt flow
Plastic Density index
Class No type (g/cm3) (g/10 min) Remarks
MFI 190/2-16
LDPE 1 aLupolen 1800 Ha 0.917-0920 1.3-1 8
2 Lupolen 3034 H 0.926-0929 1.7-2 2
3 Lupolen 1800 S 0.916-0918 17-22
4 bDowlex 2047 0.917 2.3 linear LDPE
MFI 190/2
HDPE 1 cVestolen A 3512 0.935 0.5
2 Vestolen A 6042 0.959 0.4
3 Vestolen A 3515 0.935 5
4 Vestolen A 6016 0.962 7
MFI 230/2.16
PP 1 aNovolen 1 120 HX 0.907 1.8 highly isotactic
2 Novolen 1 120 TX 0.908 37 highly isotactic
3 Novolen 1320 H 0.900 1.8 considerable atactic parts
4 Novolen 2200 KX 0.904 3.5 block copolymer with PE
MFI 200/5.0
HIPS 1 aSB 427 D 1.05 8 2nd digit of type number
(SB) 2 SB 454 C 1.05 12 indicates content of impact
3 SB 456 N 1.05 2.5 modifier: 2=lowest content
4 SB 473 D 1.05 10 7=highest content
a BASF. Ludwigshafen, FR Germany.
b Dow Chemicals, Horgen. Switzerland.
c Chemische Werke H6ls. Marl. FR Germany.
Each plastics material was ground and mixed thoroughly with 0. 1 % by weight of 14C-labelled additive, as before. The resulting basic blends were extruded and granulated and then processed, under technical processing conditions, into the following plastics products:
(a) pressed sheets,
(b) extruded sheets,
(c) injection-moulded cups,
(d) tubs deep-drawn from extruded sheets.
The quantities of additive which migrated from the different plastics products into test fat HB 307 were determined in the case of HDPE, PP and HIPS types under the standard test conditions 10 days/40°C and in case of the LDPE types at 5 days and 40°C. (Abbreviated test durations were used since preliminary trials indicated that LDPE test specimens were completely penetrated by the test fat before completion of standard test duration.) The further procedure was identical to that described above.
ResultsThe results of these migration trials are shown in tables 3 to 6. It is clear from tables 5 and 6 that the quantities of additive which migrate from various types of PP and HIPS into fat depend significantly on the processing of the plastics. Thus. independent of the type of PP or HIPS, significantly smaller quantities of additive migrated from injection-moulded cups or deep-drawn tubs into the test fat compared to the migration from the corresponding sheets. As can be seen from migration data presented in table 4, this dependency of migration rate on processing was also valid
Migrated amount of additive b (µg/dm2)
PP typea (mean and SD, n= (0) .
Melt flow index
Density (MFI 230/2.16) Remarks on Sheets Cups
No. (g/cm3) (g/10 min) the structure Pressed Extruded injection-moulded
1 0.907 1.8 highly isotactic 564 440 83.7
(24) (22) (1.1)
2 0.908 3.7 highly isotactic 635 437 210.5
(20) (15) (2.2)
3 0.900 1.8 considerable 996 1014 497.2
atactic parts (60) (61) (7.4)
4 0.904 3.5 block copolymer 720 511 125.5
with PE (82) (22) (3.2)
a As in table 2.
b Related to additive concentration in the test specimen of 0. I% by weight (compare table 3,
footnote b).
Migrated amount of additiveb (µg/dm2)
HIPS typea (mean and SD, n = 10) .
Melt flow index
Density (MFI 200/5.0) Sheets Cups Tubs
No. (g/cm3) (g/10 min) Pressed Extruded injection-moulded (deep-drawn)
1 1.05 8 4.93 4.29 1.24 0.62
(0.374) (0.150) (0.075) (0.073)
2 1.05 12 14.9 10.3 1.80 1.03
(0.86) (0.33) (0.129) (0.120)
3 1.05 2.5 12.3 8.89 1.46 0.61
(1.27) (0.287) (0.081) (0.104)
4 1.05 10 28.8 21.5 5.57 1.05
(1.53) (0.31) (0.131) (0.142)
a As in table 2.
b Related to additive concentration in the test specimen of 0 1% by weight
(compare table 3. footnote b).
(b) The least amounts of additive were transferred into the test fat from LDPE 2--the type with the highest density. Compared to LDPE 1 (a type with a distinctly lower density) about 46% less additive migrated from pressed sheets, and 37% less from extruded sheets and cups into the test fat.
Accordingly it was expected that migration trials with different types of HDPE and PP would lead to results similar to those obtained with the different types of LDPE. This assumption is confirmed in table 4 for the types of HDPE investigated. These data show:
(b) Significantly smaller quantities of additive migrated into the test fat from the HDPE types 2 and 4. These have higher densities compared to HDPE types 1 and 3. Thus, the quantity of additive which migrated from the pressed sheet of HDPE 4 (density 0962 g/cm') into the test fat is about 59% lower than the value obtained with HDPE 1 (density 0.935 g/cm3) under identical conditions.
If the migration of additive into test fat HB 307 from different types of PP, converted to tubs, pressed and extruded sheets, is observed under identical conditions (table 5) the following conclusions hold true:
(b) the migration of mobile components into fat decreases with increasing density of PP.
Generally it can be expected for all classes and types of polyolefines that the migration of components decreases with increasing density.
From the results of trials with different types of HIPS (table 6), the plastics materials with the strongest processing influence, the following statements can be made:
(b) With increasing content of impact modifier in PS, increasing amounts of the additive migrated from the pressed and extruded sheets and from the injection-moulded cups into the test fat. For instance, about six times more additive migrated from the extruded sheets prepared from HIPS 4 into test fat than from those prepared from HIPS 1.
(c) This relationship between the content of impact modifier in PS and the migration of additive into test fat HB 307 is lost to a considerable extent in the course of deep-drawing the extruded HIPS sheet. Accordingly, low and nearly equivalent quantities of additive migrated from all deep-drawn tubs prepared from HIPS 1-4.
The peculiarities in the migration of components from HIPS-a two-phase plastics material-into foodstuffs and test foods are dealt with in detail elsewhere (Klahn et al. 1983).
ConclusionsThe results of our investigations agree well with our experiences in the sector of migration and lead to the following important conclusions regarding the use of plastics in food packaging and the testing of packaging materials:
(b) Migration of components from different types of plastics. belonging to the same class, into fat-releasing contact media:
(i) clearly depends on the density of the plastic type,
(ii) in case of HIPS, also depends on its impact modifier content,
(iii) does not depend on the melt flow index of the plastic.
(c) The migration of components into fat-releasing contact media from pressed and extruded sheets prepared from different classes of plastics is in most cases higher-and in no case lower than that from the corresponding injection-moulded cups, deep-drawn tubs or blown bottles. This is valid for all types of the different classes of plastics investigated here.
References
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