PRETREATMENT OF POLYOLEFINS
The need for treatment
of polyolefins is caused by their having an inert non-polar structure that does not permit
any mechanical or chemical bonding between the moulding surface and the coating. Thus,
pretreatment can be defined as the process to which a polyolefin moulding is subjected to
prior to the application of a decorative coating (usually in ink) in order that the
applied coating will have acceptable and permanent adhesion. The most widely used
pretreatment processes are described below:
Flame treatment is
currently the most versatile method for the pretreatment of polyolefin surfaces, is almost
exclusively used for the pretreatment of blow moulded containers and is widely used for
injection moulded articles.
Flaming consists of exposing the surface to be decorated to a suitable oxidising flame for a period in the range 0.2 to 3.0 seconds. This treatment brings about a change to the polymer surface that makes it wettable and permits a strong adhesive bond between the moulding surface and the coating. This change in surface properties can be readily seen by immersing an untreated and a treated moulding in water. On removal, the water will run off in globules from the untreated moulding but will form a continuous film on the treated surface which will last for a varying length of time depending on the actual flaming conditions used.
FLAME PRETREATMENT SYSTEMS
The article may or may not rotate as it passes through the flame, and the burners may cam-follow or be placed on either side of the conveyor. This is essentially a piloted burner with a row of high velocity jets with directional force in the flame, supported by a low velocity pilot system. The general shape of the flame is similar to a single row of toothbrush tufts.
Wire mesh belt
Mouldings are laid on the belt and passed over the flame; another burner is sometimes placed above the mouldings.
This is most suitable for more manual runs, although these can also be incorporated into production lines, of cylindrical items that are simply gravity fed down a chute and through a flame ring. Items from stubby holders to pens can be flamed in this way.
FLAME TREATMENT VARIABLES
The major flame treatment parameters to be controlled if optimum flaming conditions are to be used are as follows:
(a) Gas/air ratio.
This is dependant on the type of gas used (town gas, methane, propane and butane are suitable). A laminar flow flame is preferable to turbulent flame.
(b) Distance of the burner to the moulding surface.
(c) Speed of rotation of the moulding in the flame (this is applicable to cylindrical mouldings only).
(d) Residence time of the moulding in the flame.
It has been established that the gas/air ratio is the most important and critical factor in flame pretreatment. The importance of control over this parameter cannot be overstressed and it is recommended that the gas/air flow be maintained at a pre-determined level by the attachment of rotameters to both the gas and air supply lines. It is possible to either under or over treat polyolefin surfaces, causing poor print adhesion in both instances.
Before a production run starts with a given moulding, experiments should be carried out to decide accurately the optimum flaming conditions. Once optimum conditions have been established the pretreater should be set at these conditions and routine checks made to ensure that variation does not occur.
Owing to the use of different burner designs, type and composition of gas and flame treatment equipment, it is not possible to give an exact recommendation on flaming conditions. As a result of the work carried out in the laboratory some general conditions are given below but the points made above should always be kept in mind.
Typical Flaming burner (Natural gas)
Gas/air ratio 1:8 to
Distance of burner to moulding surface 10 to 15mm
Speed of container rotation 100 to 150 rev/min
Residence time in the flame 1/2 to 3 seconds
NB. LP gas to air ratio approx. 1:20
THE EFFECT OF POLYMER VARIABLES
It has been found that no grade of high or low density polyethylene or polypropylene, whether natural or pigmented, presents any particular problem provided that optimum flaming conditions are used. The optimum conditions, while fairly critical, do not vary greatly from one grade to the next.
The effect of incorporating antistatic additives depends on the type of additive used, the concentration at which it used and the type of flame pretreater used. Additives, which by virtue of their diffusivity and compatibility characteristics, become active immediately after processing, demand more critical flaming conditions and can give rise to difficulty if used at higher concentrations than those recommended. The level of addition at which difficulty is encountered in achieving acceptable print adhesion is dependent on the type of burner used for pretreatment. Most antistatic additives are only effective after flaming, and these additives have no effect on the flaming conditions necessary for acceptable print adhesion.
Some customers maintain that the time lapse between moulding production and pretreatment has no bearing on either the conditions of pretreatment or the ultimate print adhesion obtained. Others assert that two weeks delay would be an absolute maximum whereas others still maintain the change in surface tension will alter after 1-3 hours or less from flaming affecting print quality. This time frame may be related to the type of plastic and the ink type but most likely the quality of the flaming. The best advice is to flame and print as soon as possible.
This method involves dipping the pretreated container in water and determining visually the extent and duration of the water film on the container. An untreated container repels the water immediately, whereas a treated container retains the water for up to several minutes. However, between these extremes a partially treated container will tend to show adjacent areas of good and bad adhesion. This test indicates whether or not a container has been pretreated but gives no indication of over-treatment.
Blue Ink Test (Dyne
Pretreated containers are painted with a special blue ink and left to dry. Sellotape is pressed onto the print and then pulled off rapidly; the percentage of ink removed is recorded.
Peel Adhesion Test
In this test a specified pressure-sensitive tape (3M No. 851 ) is applied to the treated surface (films or strips cut from wall of a container) using a roller. The peel strength is measured in a tensiometer. The higher the level of treatment the higher the peel strength.
Click here to return to Flame Treatment.
Copyright Australian Combustion Services Pty Ltd 1997-2004