Food contact applications stabilisers

Melt stabilisers such as phosphoric acid and its salts and esters are added to the polymerisation process to reduce thermal degradation and colour formation (yellow). They also have the added effect of stabilising the polymer when it is being later processed into food packaging. Many of the phosphorus based stabilisers are listed in 2002/72/EC for use in food contact applications and are covered by the basic polymer doctrine of the US EDA. 

Among these heat stabilisers, those containing tin (such as tin mercaptides, and tin carboxylates or maleates) are considered to be the most efficient, and can be used in a wide variety of applications. Metallic tin is harmless but there are suspicions that organotin-compounds can be toxic to the central nervous system and the liver. However, tin stabilisers have a low capacity for migrating, and hence they are still considered to be safe. Tin stabilisers such as methyl and octyl tins are used in food contact applications. For PVC, it is believed that tin stabilisers act as HCl scavengers (generating tin chloride) as well as an antioxidant. Thio-tin compounds (preferred for rigid pipe extrusions and profiles (for window frames) of PVC) may develop an odd odour due to sulfur. 

Epoxidised grades (soya bean and linseed oil) are used as stabilised plasticisers with no migration. Soya bean versions have widespread approval for food contact applications. 

Calcium-zinc stabilisers are rated as relatively nontoxic and are favoured for use in medical plastics, toys and food contact applications. They can be formulated as one-pack systems along with other additives to minimise handling problems. They normally have a self-lubricating role... 

Traditional substances used as co-stabilisers include polyols, epoxidised soya bean oil, phosphites and certain antioxidants. Arkema markets Stavinor calcium stearate co-stabilisers for use with calcimn-zinc systems. Akcros Chemicals sells P24D, which stands for pyrrolidene-2,4-dione. Siid-Chemie is now marketing a synthetic hydrotalcite, Sorbacid 911, while the PQ Corporation is selling two grades of aluminosilicate (zeolite) imder the name Advera 401. They act as scavengers for acids and metal chlorides, and enable users of tin mercaptides to use less tin, saving on material costs. They also allow stabiUsation to take place with the very low tin levels required by law in food contact applications. 

Stabilisation with antioxidants may render PP unsuitable for food contact application as they may directly migrate into the food products, hydrolysing and imparting odour or taste to the food. Careful selection of a suitable stabiliser system is necessary for food contact applications. In addition, heat stabilisers could adversely affect the working of light stabilisers. 

Secondary antioxidants or peroxide decomposers such as phosphites and thioesters inhibit oxidation. They are usually combined with primary antioxidants to produce synergistic effects. Careful selection of suitable stabilisers is necessary for food contact applications. Interaction with light stabilisers and other additives must be considered during selection (see Chapter 4). 

Flame retardants are added to reduce the flammability of PE. The unmodified PE grades have a 94 horizontal burn rating up to 0.8 mm thickness. Suitable flame-retardant grades with V-O rating at 3.2 mm thickness are available from compounders. Flame retardants can reduce the processability and interfere with the function of certain hindered amines as light stabilisers. Flame-retardant grades are generally not suitable for use in food contact applications. 

Polyvinylchloride (PVC) is much less crystalline than the polyolefins. The base polymer is very hard and for most applications it has to be plasticised to make it flexible enough for use. There are many plasticisers available for PVC. Food contact applications, of course, impose their own requirements of intrinsic safety and low migration rate of the plasticiser. Due to the nearness of its decomposition and processing temperatures, PVC has to be stabilised against heat by the addition of 1-2% of heat stabiliser and this, in turn, has food contact implications. 

ESBO is a heat stabiliser and secondary plasticiser used in PVC-P applications. It has food contact approval up to certain limits but there have been some environmental/political issues concerning the possible use of genetically modified soya bean oil in the manufacture of ESBO. 

Among recent developments, there is the stabiliser of high-purity tris-nonylphenyl phosphite, with 0.1% residual nonyl phenol, which is FDA-approved for food-contact (and medical) applications. 

Despite the large number of PVC stabiliser types mentioned in Chapter 4, PVC applications still pose difficulties. Food contact packaging or medical device manufacturers in particular have a rather limited choice of satisfactory stabilisers. European pipe fitting manufacturers have also reported some difficulties. 

An alternative approach for producing latex with a wide particle size distribution is microsuspension polymerisation. In this process, an initiator such as lauroyl peroxide is used, which is highly soluble in the VCM, but is essentially insoluble in water. Thus, polymerisation takes place within the dispersed VCM droplets. The water insolubility of the initiator also helps to stabilise the VCM droplet, and it may be possible to use lower levels of emulsifier compared with the batch emulsion and continuous emulsion processes. Lower levels of emulsifier can be advantageous, for example for applications coming into contact with food, where water absorption or clarity is important, and also for the environmental impact of the proccess. Such latexes produce polymers which give very low plastisol viscosities, but tend to be dilatant in nature. This can be overcome by modifying the process to have a secondary particle size distribution alongside the primary one. 

When the intended application involves contact with food or drugs, then the base elastomer is selected from a small number of butyl grades. These are grades from which contaminants have been virtually eliminated and which either do not contain a stabiliser, or which contain one that is sanctioned by the appropriate regulatory authorities. For other types of application, however, selection is normally made on the basis of the level of unsaturation, the viscosity of the elastomer, and whether or not its stabiliser is staining or discolouring. 

Synthetic high c/5-polyisoprenes which are water-white in colour are stabilised with non-toxic antioxidants which conform to existing legislation on materials suitable for contact with food and drugs. Since they also have extremely low levels of impurities this makes them attractive polymers for pharmaceutical applications. 

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