Properties and composition of plastic FCMs
Plastics can be placed into two main categories, thermoplastic and thermoset. Thermoset plastics are irreversibly formed into a permanent shape often by applying heat. Thermosets cannot be softened and remoulded on heating and have few applications in food packaging, except for the inner linings used for can coatings and many adhesives, as used, for example, in multilayer materials. A limited range of food contact materials is made from thermosets, predominantly melamine resins and unsaturated polyesters used in tableware and utensils.
Thermoplastics can be softened repeatedly by heating and are more easily recyclable. These plastics are used most often in food contact applications and will be considered in most detail in this chapter. Cellophane, an important material used for packaging of sweets and pies, made from cellulose, is strictly not considered to be a true plastic. Its versatility has been increased by the additions of softeners and polymeric coatings. Food packaging materials tend to be grouped into two categories in market research, flexible and rigid. The main types of plastics and other materials used as substrates in flexible packaging applications are given in Fig. 10.3. For rigid packaging materials, a significantly higher proportion of PET, as used in beverage bottles, and polystyrene, used for expanded polystyrene food trays and high impact polystyrene cups, and pots is used.
The criteria for selection of the most appropriate polymeric materials for food contact applications are generally based upon:
• physical properties at different humidities and temperatures (-18 °C for frozen foods to 200 °C+ for ovenable foods), including dimensional stability
• inertness to foods under given use/storage/processing/cooking temperatures
• ease of conversion
• clarity, visual appeal and printability.
Further factors sometimes come into play for food packaging:
• flexibility
• sealability
Fig. 10.3 Flexible packaging substrate use 2003 (reproduced with permission from Research Information Ltd).
Fig. 10.3 Flexible packaging substrate use 2003 (reproduced with permission from Research Information Ltd).
• barrier properties to oxygen and water (in some cases also carbon dioxide and ethylene).
These criteria are not in any particular order and some may conflict with one another. Fortunately for food packaging, in cases where no single plastics monolayer can satisfy all the functional requirements, recently applied technologies such as co-extrusion and lamination can provide complete solutions. The most commonly used plastics for the manufacture of FCMs are given below.
10.3.1 Low density polyethylene (LDPE)
This type of plastic is used frequently for food bags, which are produced by blowing a film. As LDPE is very flexible it is also used to make lids for food storage containers, produced by injection moulding techniques. The polymer is relatively cheap with good water vapour and moisture resistance, but has poor barrier properties to gases and low molecular weight organic chemicals. 'Stretch films' used for wrapping food contain a few percent of a polybutadiene additive to provide a degree of 'cling' property. LDPE is often used as a film or coating on other materials, such as paper and aluminium foils to provide flexibility and heat sealability.
Ethylene may be copolymerised with vinyl acetate to make ethyl-vinyl acetate, offering high seal integrity and clarity for frozen food applications where a high degree of toughness is required. Ethylene copolymers with other olefins such as propylene, 1-hexene and 1-octene allow a range of properties to be achieved. Linear low density polyethylene (LLDPE) has a linear structure when compared to LDPE which has a high degree of branching. Frozen food, for example, for meat and poultry, must maintain strength at very low temperatures. LLDPE is one material that meets this requirement, although a traditional disadvantage of LLDPE has been its lower clarity than LDPE. This has largely been overcome by metallocene-catalysed grades which also offer greater strength and better oxygen barrier properties. Copolymers with acrylates produce ionomers which have superior heat sealability and are increasingly used in laminated films.
Polyethylenes are susceptible to oxidative degradation at processing temperatures, therefore antioxidants are added. Silica is sometimes added to LDPE as an antiblock to prevent films sticking and N,N-bis(2-hydroxyethyl)alkyl(C8-C18) amine (BEA) as an additive to reduce a build up of static charge (antistat). Table 10.2 gives commonly used substances in polyethylenes.
10.3.2 High density polyethylene (HDPE)
HDPE is used in similar applications to LDPE, but has better barrier properties than LDPE and a greater rigidity. So HDPE is also used for blow-moulded bottles for milk and other drinks, and in food storage containers. HDPE has a higher usable temperature than LDPE (up to about 100_120 °C) making it suitable for 'hot fill' and pasteurisation applications. HDPE is used for meat and poultry packaging because of its greater strength and puncture resistance at thinner gauges. Another increasing market for HDPE is in closures for
|
Substance name |
PM ref. no. |
Function |
Restriction SML |
|
Ethylene |
16950 |
Monomer | |
|
Propylene |
23980 |
Comonomer | |
|
4-methylpentene |
22150 |
Comonomer |
0.05 mg/kg |
|
1-hexene |
18820 |
Comonomer |
3 mg/kg |
|
1-butene |
13870 |
Comonomer | |
|
1-octene |
22660 |
Comonomer |
15 mg/kg |
|
Pentaerythritol tetrakis [3-(3,5-di- |
71680 |
Antioxidant | |
|
tertbutyl-4-hydroxyphenyl)propionate] | |||
|
Phosphorous acid, tris(2,4-di-tert- |
74240 |
Antioxidant | |
|
butylphenyl)ester | |||
|
Octadecyl-3-(3,5-di-tert-butyl-4- |
68320 |
Antioxidant |
6 mg/kg |
|
hydroxyphenyl)propionate | |||
|
Erucamide, oleamide, stearamide |
52720, 68960, 88960 |
Slip additives | |
|
N,N-bis(2-hydroxyethyl)alkyl |
39090 |
Antistatic |
(T) 1.2 mg/kg |
|
(C8-C18) amine | |||
|
Glycerol stearate |
57520 |
Antiblock | |
|
Silica (silicon dioxide) |
86240 |
Antiblock |
beverages. The monomers and additives used in HDPE formulations are similar to those in LDPE. A typical formulation for a HDPE bottle would be: HDPE 100 parts per hundred (pph); octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate 0.05 pph; phosphorous acid, tris(2,4-di-tert-butylphenyl)ester 0.1 pph.
10.3.3 Polypropylene (PP)
The market for polypropylene polymers and copolymers is generally increasing owing to their excellent versatility in a range of food processing conditions. Generally, PP is stiffer than LDPE or HDPE and has superior tensile strength, good clarity and grease resistance. Co-polymerisation of propylene with ethylene and other olefins yields materials with greater flexibility and impact strength, fulfilling requirements for low temperature applications. PP can be converted by a range of procedures to make films, pouches, closures, containers, bottles and injection moulded containers and articles that can withstand retorting and microwave reheating. Similar conversion methods are used as with other polyolefins, but with the addition of thermoforming to make, for example, trays for cakes and pots for yoghurts.
A typical formulation for a polypropylene (PP) film used for wrapping biscuits would be: PP 100 pph; pentaerythritol tetrakis[3-(3,5-di-tertbutyl-4-hydroxyphenyl) propionate] 0.1 pph; phosphorous acid, tris(2,4-di-tert-butylphenyl)ester 0.1 pph; erucamide 0.05 pph. Nucleators, such as substituted benzylidene sorbitols, can be added to improve the clarity of PP and LDPE (see Fig. 10.4). Oriented polypropylene (OPP) films are also available where a higher transparency is required with improved strength.
10.3.4 Polystyrene (PS)
Polystyrene homopolymer, often referred to as general purpose polystyrene (GPPS) or 'crystal' polystyrene, is a hard, fairly brittle polymer with excellent transparency. GPPS is used for making disposable tableware and plastic glasses. Styrene polymers and copolymers with butadiene and acrylonitrile, to a lesser extent, are used in a range of food contact applications including tableware, wine/beer glasses, yoghurt pots, coffee cups, and thermoformed trays for meat and fish. Although GPPS is hard it is rather brittle and lacks strength, therefore impact modifiers and/or copolymerisation with 1,3-
Fig. 10.4 Structure of bis(3,4-dimethylbenzylidene)sorbitol PM ref. no. 38879.
Fig. 10.4 Structure of bis(3,4-dimethylbenzylidene)sorbitol PM ref. no. 38879.
butadiene, to varying degrees, provide increased flexibility and strength. Oriented polystyrene (OPS) retains the clarity but increases the strength of the material and is better suited to making transparent films.
Expanded polystyrene (EPS) articles are made using high impact polystyrene formulations with incorporation of a blowing agent (this is usually a volatile solvent such as pentane). Often, EPS containers will have a crystal polystyrene skin applied to the food contact surface to act as a barrier between food and container. Applications of EPS include thermoformed packaging for eggs, meat, fish and fast food trays. Substances typically used in the manufacture of polystyrene are given in Table 10.3.
10.3.5 Polyvinyl chloride (PVC)
Vinyl chloride can be polymerised to form polyvinyl chloride (PVC) which is fairly brittle and unsuitable for food contact applications, so it is mixed with plasticisers to soften the polymer and impart flexibility. Plasticised PVC may contain about 30% of plasticisers and is used to make stretch films and flexible PVC. Flexible PVC used for tubing and gaskets may contain di(2-ethylhexyl)phthalate, and stretch films will probably contain di(2-ethylhexyl)adipate and a polymeric adipate plasticiser. 'Rigid' PVC may
|
Substance name |
PM ref. no. |
Function |
Restriction SML |
|
Styrene |
24610 |
Monomer | |
|
1,3-Butadiene |
13630 |
Comonomer |
QM 1 mg/kg SM = ND, DL = 0.02 mg/kg |
|
Acrylonitrile |
12100 |
Comonomer |
SM = ND, DL = 0.02 mg/kg |
|
'White' mineral oil |
95883 |
Extender |
Specification |
|
Pentaerythritol tetrakis [3-(3,5-di- |
71680 |
Antioxidant | |
|
tertbutyl-4-hydroxyphenyl)propionate] | |||
|
Phosphorous acid, tris(2,4-di-tert- |
74240 |
Antioxidant | |
|
butylphenyl)ester | |||
|
Octadecyl-3-(3,5-di-tert-butyl-4- |
68320 |
Antioxidant |
6 mg/kg |
|
hydroxyphenyl)propionate | |||
|
Erucamide, oleamide, stearamide |
52720, 68960, 88960 |
Slip additive | |
|
N,N-bis(2-hydroxyethyl) |
39090 |
Antistatic |
(T) 1.2 mg/kg |
|
alkyl(C8-C18) amine | |||
|
Thiodipropanoic acid, didodecyl |
93120 |
Antioxidant |
(T) 5 mg/kg |
|
ester (DLTDP) | |||
|
Thiodipropanoic acid, dioctadecyl |
93280 |
Antioxidant |
(T) 5 mg/kg |
|
ester (DSTDP) | |||
|
Alkyl (C8-C22) sulphonic acids |
34230 |
Antistatic |
6 mg/kg |
contain low levels of plasticiser and is used to make trays for fresh meat having good clarity and water bottles. PVC compounds are calendared - the resin is forced between rollers to form a sheet that can then be thermoformed to produce trays or pots. PVC bottles are blown in a similar way to HDPE.
PVC tends to degrade slowly at processing temperatures to release hydrogen chloride (HCl) which can impair the performance of the plastic. Therefore HCl scavengers are added to the PVC compound. ESBO (epoxidised soybean oil) acts as both an HCl scavenger and a plasticiser, and is used in both stretch films (with other plasticisers) and in cap gaskets. In rigid PVC formulations, tin stabilisers are often added, to scavenge HCl, in conjunction with calcium/zinc stearates. One limitation of rigid PVC is that its maximum use temperature is about 70 °C, above which it begins to deform. Copolymerisation of vinyl chloride with vinylidene chloride increases this temperature so that a film suitable for microwaveable reheating is produced. Polyvinylidene chloride (PVDC) film is commonly plasticised using a plasticiser such as tri-n-butyl acetyl citrate (ATBC). Vinyl chloride monomer (VCM) is the subject of three directives: 78/142/EEC, 80/766/EEC and 81/ 432/EEC where the limits for VCM are given together with analytical methodology. Table 10.4 lists commonly used substances in PVC.
10.3.6 Polyethylene terephthalate (PET)
PET is made by polymerising ethylene glycol with terephthalic acid or transesterification with dimethyl terephthalate, commonly using an antimony trioxide catalyst. Other monomers such as isophthalic acid are used to make copolymers for some bottle formulations. The use of PET plastics has increased significantly over the last ten years replacing glass and, to some extent PVC, in applications for water and soft drinks. PET bottles are made in a two-stage process, initially a preform is made which is then blow moulded. PET is also thermoformed to make trays and pots used for cooking and heating foods in both conventional and microwave ovens. The PET used in high-temperature applications has a higher crystallinity and is opaque compared to the transparent amorphous material used to make bottles. Greater crystallinity is achieved by heating and/or addition of crystallisation aids. By-products of polymerising or processing the components of PET are diethylene glycol and acetaldehyde, although acetaldehyde scavengers, such as anthranalamide (2-aminobenzamide), can be used to reduce the level of acetaldehyde which can cause taint/odour problems.
Polyethylene naphthalate (PEN) polyesters are made from 2,6-naphthalene dicarboxylic acid or 2,6-naphthalene dicarboxylic acid, dimethyl ester. They have higher temperature resistance than amorphous PET and are increasingly used in applications requiring heat sterilisation of the food/drink, although PEN at the moment is significantly more expensive. Table 10.5 lists commonly used substances in polyesters.
|
Substance name |
PM ref. no. |
Function |
Restriction SML |
|
Vinyl chloride |
26050 |
Monomer |
QM = 1 mg/kg |
|
or SML = ND | |||
|
(DL = 0.01 | |||
|
mg/kg) | |||
|
Acetic acid, vinyl ester |
10120 |
Comonomer |
12 mg/kg) |
|
Vinylidene chloride (for PVDC) |
26110 |
Comonomer |
QM = 5 mg/kg |
|
or SML = ND | |||
|
(DL = 0.05 | |||
|
mg/kg) | |||
|
Methacrylate/butadiene/styrene (MBS) |
Impact |
See Table 10.3 | |
|
modifier | |||
|
Acrylic polymer |
Process aid | ||
|
Octadecyl-3-(3,5-di-tert-butyl-4- |
68320 |
Antioxidant |
6 mg/kg |
|
hydroxyphenyl)propionate | |||
|
Erucamide, oleamide, stearamide |
52720, |
Slip additive | |
|
68960, | |||
|
88960 | |||
|
N,N-bis(2-hydroxyethyl)alkyl |
39090 |
Antistatic |
1.2 mg/kg |
|
(C8-C18) amine | |||
|
Soybean oil, epoxidised (ESBO) |
88640 |
Stabiliser |
60 mg/kg |
|
30 mg/kg | |||
|
(infants) | |||
|
Specification | |||
|
Adipic acid, bis(2-ethylhexyl)ester |
31920 |
Plasticiser |
18 mg/kg |
|
(DEHA) | |||
|
Phthalic acid, bis(2-ethylhexyl)ester |
74640 |
Plasticiser |
3 mg/kg under |
|
review | |||
|
Polyester of 1,2-propanediol and/or |
76866 |
Plasticiser |
30 mg/kg |
|
1,3- or 1,4-butanediol | |||
|
Calcium/zinc stearate or laurate |
Stabiliser | ||
|
Di-ra-octyl-tin compounds |
Stabiliser |
SML (T) | |
|
0.006 mg/kg as | |||
|
total tin | |||
|
Mono-ra-octyl-tin compounds |
Stabiliser |
SML (T) | |
|
1.2 mg/kg as | |||
|
total tin | |||
|
Methyl tin compounds |
Stabiliser |
SML (T) | |
|
0.18 mg/kg as | |||
|
total tin | |||
|
Glycerol stearate |
56585 |
Lubricant |
(T) = total migration of two or more moieties.
(T) = total migration of two or more moieties.
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