Лако-красочные материалы — производство

This class of plastic is far more important to the plastics coating sector, in respect of the tonnage used, and the diversity of the polymer types employed. The following thermoplastics account for the majority of moulded articles which are subsequently coated.

ABS (Acrylonitrile-butadiene-styrene) — is a high performance commodity plastic used particularly in the automotive industry where high impact strength is required. It exhibits a high level of tensile strength and rigidity, but is degraded by ultraviolet light.

PMMA (Acrylic, also known as Perspex) — is often used for its optical properties. Polymethyl methacrylate absorbs very little incident light, infact the light transmission through a parallel sheet free from faults is about 92%. The mar resistance of acrylic polymers is poor and is often upgraded through the use of a clear hardcoat.

PVC (Polyvinyl chloride) — is a very flexible substrate often used in the production of imitation leathercloth and flooring. It suffers badly from UV degradation and plasticiser migration. Both properties can be upgraded with a suitable coating.

HIPS (High impact polystyrene) — the major application for this plastic is in the packaging industry, where low cost and rigidity are economically and technically vital. The material has a tendency to yellow and shows very poor mar resistance. The most important application for the coating supplier is the widespread use of this plastic in the consumer electronics industry. Electrical insulation properties are good and cost is low.

PPO (Polyphenylene oxide) — is often discovered under the trade mark "Noryl". It is an engineering polymer which has superior physical properties and temperature resistance when compared with ABS. It suffers some UV degradation but has nonetheless found widespread use in the personal computer industry.

PC (Polycarbonate) — the major use for this polymer is in the manufacture of synthetic glazing. It does, however, require protection from UV light to extend the lifespan of the glazing unit. It has superior heat resistance, tensile strength and impact resistance when compared to the other engineering polymers, but can be very sensitive to strong organic solvents.

PA (Polyamide) — the nylons have several major advantages over other thermoplastics, being insensitive to most organic solvents and stable when exposed to high temperatures. This latter property allows the use of stoving acrylic systems with good gloss and high adhesive strength. The major disadvantage is that the nylons are hygroscopic. This water absorption has a plasticising effect, causing a reduction in tensile strength.

PP (Polypropylene) — the use of this polymer has recently become far more widespread because it can now be satisfactorily coated following surface pretreatment. It is resistant to most organic solvents and is stable to relatively high temperatures. This makes the use of low-bake convertible coatings possible.

In recent years new polymers and alloys have been regularly introduced, creating new challenges for the coating technologist.

Plastic substrates require special consideration because of four major concems:-

1. Heat sensitivity.

2. Low energy surface.

3. Solvent sensitivity.

4. Flexibility

The distortion temperature of the polymer must not be exceeded (see table below). Materials such as high impact polystyrene have a Tg (glass transition temperature) of 80°C, which severely restricts the choice of a coating system.

The low energy surface of some polymers can create problems with adhesion. Critical surface tension values for some common materials are given in the following table. Substrates such as the polyolefins, polypropylene and polyethylene are extremely difficult surfaces on which to achieve good adhesion. Techniques such as flame treatment have been developed, which oxidise the surface of the component, but these still tend to be used in conjunction with the standard chlorinated polyolefin primer in a “belt and braces” approach to adhesion. The most reliable form of polyolefin pretreatment is plasma surface treatment.

It is a commonly held belief that solvent attack is the means by which adhesion to plastic substrates is achieved. In a handful of cases the addition of an aggressive solvent will greatly enhance the adhesion of a coating system. PVC is a prime example. In an acrylic coating system, the addition of five to ten percent cyclohexanone makes the difference between pass and failure when tested for adhesion. The more likely explanation is that the aggressive solvent, in solvating the surface layer of the plastic, modifies the surface tension at the interface5. The molecules contributing to the adhesive bond (believed to be formed by Van der Waals forces) are then able to approach the substrate intimately. All intermolecular attractions operate over extremely short distances, i. e. < 1 nm. At distances above 0.1 nm, their contribution to adhesion is negligible. This technique of aggressive solvent addition should always be carefully evaluated. This will ensure that problems, such as crazing or impact resistance reduction of the component, do not occur on ageing.

The sensitivity of thermoplastics to solvents covers both ends of the spectrum. Materials such as nylon are resistant to most solvents and are only attacked by strong acids. At the other extreme high impact polystyrene is sensitive to most common solvents, excluding the lower alcohols, glycol ethers, the lactates and some aliphatic hydrocarbons (aromatic content can cause problems). Solvent balance is critical to formulating a satisfactory coating for this plastic. The tolerance of acrylic polymers to aliphatic hydrocarbons and alcohols is very useful for solvent sensitive plastics. This tolerance increases with the length of the side chain, thus butyl methacrylate homopolymers are soluble in various grades of mineral spirits. Methyl methacrylate homopolymers’ limited solubility and compatibility mean that their usage is mainly restricted to high specification automotive systems on less solvent sensitive substrates. The most widely used group of acrylics is the butyl methacrylate / methyl methacrylate copolymers. Their compatibility with other resins is good, tolerance of mil<J organic solvents is excellent and they have some inherent flexibility.

Ethylene glycol mono butyl ether was historically a favourite thinner for plastic coatings, having little or no effect as a solvent on sensitive polymers. In recent years the health and safety questions concerning the use of this solvent have required formulators to think twice about its usage. The sensitivity issue creates a major problem for the coating manufacturing plant, contamination of a thinner with a few percent of aromatic hydrocarbon could mean the difference between a successful product and one that will attack every fault in the moulding. If in doubt GLC analysis of the thinners confirms the formulation.

It is also important to match the flexibility of the acrylic with that of the substrate to be coated and to ensure that good adhesion is obtained. As a general rule, low molecular weight acrylics are recommended for general purpose coatings where flexibility and gloss are required. The medium and medium high molecular weight acrylics are used where the maximum abrasion and chemical resistance are specified. For the majority of applications the acrylic system is modified. This will be discussed in more detail later.