Лако-красочные материалы — производство

Epoxy phenolic or PVC containing solvent based lacquers have traditionally been used for internally lacquering food cans. Not all food cans are internally lacquered, however, it is normal practice to lacquer the ends of three piece cans if they are made from TFS (tin free steel). Three piece can bodies are made from ETP because of the difficulties in welding TFS. ETP is inherently more resistant to corrosion than TFS, thus for low aggressive foodstuffs lacquering is not essential but may be done for cosmetic purposes. TTS requires protection, both internally and externally. Cooking the food in the can may subject the internal lacquer to very aggressive environments, depending upon the contents of the can (pack). Many three piece food cans are not externally decorated, but have a paper sleeve. The characteristic ‘gold’ colour on the ends of the can is the result of the epoxy phenolic lacquer crosslinking.

The approach to water based lacquers for food cans has generally been of trying to upgrade the performance of lacquers based upon resins used for DWI internals, which is a logical and rational route. Other routes to water based food internals have also been, and are being, evaluated. Whilst there have been many trials, there are only limited quantities of water based internal food lacquers being run commercially. It is anticipated that this will eventually change. Corrosion resistance and ease of roller application are still major problems to overcome. The easier to pack products (low aggressivity for lacquers and metals) will undoubtedly be the first cans to utilise water based lacquer technology. It should be realised that some flexibility is required from the lacquer, even for a three piece can. To reduce costs less metal is used on the body of the can. Beading (rings around the can body) is now used to give a lower weight can, but with the same strength of a higher weight can. Ends are stamped out of sheet and contain expansion rings (ridges) to allow for expansion during cooking and contraction during cooling.

One major difference between beer and beverage DWI cans and food cans is that food cans are filled under vacuum (to exclude oxygen) and must be strong enough to withstand atmospheric pressure, whilst beverage cans are filled under pressure, thus they can be of a thinner gauge metal. Food tends to be cooked in the can.

There are many other problem M’eas that internal food can lacquers are subjected to, including sulphur staining and odour generation arising from hydrogen sulphide generation as a result of cooking protein containing foods. Overcoming these problems is at the limit of current solvent based technology, thus water based technology for food can internals still has a long way to go.

Closely allied with water based food internals are water based aerosol internal lacquers and many of the water based developments will parallel one another.

One of the most demanding apphcations for any internal lacquer is in deep drawn food cans. Here the sheet is coated and then deformed to give a food can. The internal lacquer must withstand the tremendous deformation and still possess all of the other properties required. At this point in time, no water based lacquer will successfully withstand this deep drawing operation and still perform.

Consider now a starting point formulation for a sheet fed food can internal. The epoxy acrylic discussed earlier (epoxy resin 1 , Chapter II § VII. 1) will be used.

(i) Sheet Fed Water Based Lacquer for Food Can Internals

Viscosity 165 cPs @ 25°C

nvc 40% 193°С for 10 minutes

Cymel is the crosslinking resin. The above formulation can be used as a coating for roller coat application in a sheet fed process for food can manufacture. The formulation exhibits excellent adhesion over TFS, plus good acid and alkali resistance. Again, as for DWI internal lacquers, if required the coating viscosity/solids relationship can be modified by the DlW/tertiary amine balance. The coating rheology of this formulation could also be modified by addition of water soluble polymers, or associative thickeners in order to produce a coil coating material. The practical limits to the type and quantity of the solvents and additives used are created by legislative and commercial factors, i. e. toxicity, maximum VOC allowable and the price of the formulated lacquer.

One of the major obstacles to the introduction of water based internal lacquers for food cans is the limited number of raw materials which can be used due to FDA and other local government legislation for direct food contact materials compliance.

XIV

[1] = 186

Molecular weight of styrene monomer = 104.

Therefore molecular weight of styrene polymer = 186 x 104 = 19,000. For termination by disproportionation, I=v.

The role of a surfactant in emulsion polymerisation may vary during the course of the polymerisation. Initially the surfactant contributes to the rate of polymerisation and particle formation. Once polymer is present the surfactant has to solubilise the polymer preventing precipitation and when polymerisation is complete the surfactant is required to stabilise the emulsion preventing flocculation of the polymer and the formatio; of aggregates. Where the product is utilised in latex form the surfactant plays a major part in the performance characteristics and in particular in the areas of:

• freeze thaw stability

• water sensitivity

• mechanical stability

• corrosion resistance

[4] gloss

The relationship between surfactants and particle size of the emulsion is of paramount importance. In general, increasing surfactant level decreases particle size. However, the relationship is an exponential one, and each surfactant type has a maximum level above which the particle size does not decrease.

Commercially available surfactants are rarely pure compounds. They often contain by-products of their formation reactions and often there is no direct replacement of one surfactant by a similar type of surfactant from a different manufacturer. In this section it is only intended to provide a brief background to the types of surfactant available. Comprehensive lists of surfactants detailing trade name, chemical type and structure are available from British, European and American surfactant manufacturers, as well as from independent catalogues.

[5] The dry film must be non toxic, and meet food contact legislation.

2. The dry film must have the chemical resistance to provide a protective barrier between the packaged product and the packaging.

3. The dry film must have the physical properties to impart excellent flexibility and, of course, adhesion to the substrate.

4. The dry film must not impart any flavours to or remove any flavour from the packaged product.

[6] Cellulose acetate butyrate (CAB) — This is often used to improve the sprayability of high molecular weight methyl methacrylate homopolymers, thus reducing the

[7] Resistance to mould growth — usually better than water-borne latex systems.

• Hydrolytic stability.

• Breath-ability — the humidity resident in walls and plaster must be allowed to be released otherwise blistering will result. Acrylic based coatings can be formulated to be micro-porous.

[8]NOTE: These are thixotropic resins used to reduce film sagging in high solids automotive paints.

[9] better flexibility/hardness combination (no acrylic has yet been formulated for spinspin necking, which is the most severe DWI necking process)

• lower odour

• much better impact resistance.

[10]

[11] Thermoset acrylic systems — where restrictions in molecular weight during their solution polymerisation phase, are overcome when they crosslink.

[12] 42% solids emulsion.