GB2272448A - An improved method of manufacturing opaque coloured plastics material for use in oil tanks - Google Patents

Abstract

A method of manufacturing a coloured plastics material formulation for moulding comprises the steps of compounding pigment with molten granules of plastics material, grinding said pigmented material into a powder form, and mixing said pigmented powder with unpigmented powder of similar plastics material. The formulation may be used for moulding coloured oil storage tanks. Ideally the formulation comprises 50 - 90 wt.% non-pigmented plastics powder and 10 - 50 wt.% pigmented plastics powder.

Description

AN IMPROVED METHOD OF MANUFACTURING OPAQUE COLOURED PLASTICS MATERIAL FOR USE IN OIL TANKS The present invention relates to a method of manufacturing coloured plastics material, particularly for use in oil storage tanks widely used for the storage of domestic heating oils in central heating systems such as kerosene, paraffin and diesel fuels.

The invention will be described with reference to oil storage tanks, although it is not limited thereto.

The thermoplastics material used is normally a medium to high density linear polyethylene and the method of moulding employs a casting technique known as rotational moulding. It is important that tanks made by this method are coloured, and have excellent low temperature impact properties, as they are transported, installed and used outdoors in all weather conditions..

The current method of manufacture is to grind, and so reduce, unpigmented natural polyethylene granules to a powdered state where the mean particle size is approximately 300 microns and the maximum size of any particle is in the region of 500 to 600 microns.

Natural or uncoloured tanks can then be manufactured by the well-established rotational moulding or rotational casting process in which a rotating hollow mould containing the thermoplastic powder is heated so that the particles are distributed evenly within the mould, soften and eventually melt to form a homogeneous molten layer against the inner mould surface, at which time the mould and its contents are cooled. Once cooled the mould is opened and the hollow moulded tank removed.

To colour the tank it is common practice to pre-blend the unpigmented thermoplastic powder with dry powdered pigments and to then charge the mould with the resultant dry blend of thermoplastic powder and pigments.

This method is a low cost colouring system that produces a moulded tank of even colour.

It is, however, necessary to add sufficient pigment to the blend to effectively reduce the transmission of light through the walls of the tank so that the oil in the tank is protected from the harmful effects of UV radiation.

The burn efficiency of the oil burning boiler system can be effected by the degradation of the oil which can form low molecular paraffinic, gum-like substances when attacked by light.

A disadvantage of this manufacturing method is that the pigment particles, which are very finely divided powders of particle size well below that of the ground thermoplastic (i.e. approximately 10-100 millimicrons) will naturally coat the surfaces of the thermoplastic powder particles and, remaining solid throughout the heating process, interfere with the necessary fusion of the thermoplastic powder particles to each other.

This is clearly visible when a thin section of the moulding is examined at X25 magnification (See Figure 1.) The degree of interference to proper fusion is dependent on the amount of pigment present in the blend Clearly a low degree of pigmentation will result in some thermoplastic particles remaining unpigmented or only partially coated, thus allowing some interparticle fusion to take place. Higher levels of pigmentation will increase particle surface coating and prevent adequate fusion. Much work has been done by several researchers as to the level of pigment present in the blend that could result in a dangerous reduction in the physical properties of the tank wall, particularly when measured by impact at low temperatures.It is universally accepted that the pigment concentration in the finished blend should be no greater than 0.25% as a significant drop in low temperature impact is recorded at this level.

With very dark pigments such as black (carbon black), it is possible to reduce adequately light transmission through the tank at concentrations of less than 0.25%, but for colours other than black, say green, it is necessary to use at least 0.25% pigment and usually concentrations between 0.3 and 0.5% are used, which have a significant effect on low temperature impact properties.

It has been suggested that the degree of dispersion of the pigment when dry blending is probably significant in achieving the best possible impact properties. A high speed mixing action is a system that is more cost effective than the common low speed tumble mixing method, but it can be seen that this more efficient dry mixing system merely results in a more effective surface coating of the thermoplastic powder particles and can, as a result, create greater interference to particle fusion with even greater reduction in impact properties.

Another disadvantage of this process is that the pigments tend to separate and rise to the inner surface of the tank walls whilst the powder is being tumbled in the rotating mould. The effect is that the inner surface of the finished tank can be pigment rich with a concentration much greater than was originally intended overall. This has a further effect of reducing the physical properties of the tank. Above 0.25% pigment concentration, it is possible to wipe pure pigment off the inner surface of the tank which can contaminate its contents.

In addition, the use of very finely divided pigment in a moulding shop produces its own health hazards. Operators working with dry pigments must be suitably masked to avoid inhalation of this extremely finely divided material.

Furthermore, free pigment which has not been fully encapsulated in the polymer during melting has caused concern in a number of quarters and certain types of pigments have been withdrawn by the manufacturers because the effect on health is not fully understood. One family of pigments, diarylides, are said to produce decomposition products under these conditions that could be carcinogenic.

The true cost of using dry pigment blends is not simply the cost of the raw ingredients. Apart from the foregoing, there is also the effect that loose pigment has on the metal surfaces of the mould. Migration of the pigment occurs and the moulds become stained and in time, severely fouled, to such a degree that non-pigmented tanks or tanks of another colour cannot be made without thorough cleaning and resurfacing.

All tank moulds also need to be coated with a mould release agent to ease part extraction. Pigment contamination of the surfaces reduces both the effectiveness of this release and the service life of the coating making it necessary to recoat the mould more frequently than normal.

The alternative to this process is to use a fully compounded coloured material where the thermoplastic granules are melted in a melt compounding machine with efficient melt mixing capability such as a screw extruder and then to add the desired level of pigment into the melt where it is evenly dispersed and fully encapsulated. The coloured molten extrudate is cooled, regranulated and then fed to a grinding machine to produce the coarse powders necessary for the casting process. By this means the individual thermoplastic powder particles all contain within them the desired level of pigmentation (0.1 to 0.3%) and fusion between these particles is not impeded by high concentrations of dry pigment at the surface.

The low temperature impact properties of a tank made in this way are excellent and not greatly different from that of a natural unpigmented tank.

It is, however, a fact that the melt compouding process required to achieve this result is an extra cost which is not considered acceptable to the maufacturers of rotationally moulded tanks.

An improved method of colouring oil tanks is clearly required and this has been achieved as follows: The melt compounding system, such as melt extrusion or some other melt mixing method, is used to produce a fully compounded mixture such as described above and the resultant coloured granules are then reduced to a powder of suitable size for casting.

It is not sufficient merely to blend a natural powder with a colour compounded powder in order to cut costs. Such a blend will appear to have lost some of its colour intensity and will allow light transmission. For a given colour requirement, the coloured powder particles could have an increased pigment content over that of unblended compounded powder. However, the pigment concentration cannot be too high as this will result in a reduction in impact properties of the tank.

The critical level at which impact properties are effected is in the region of 1%, whilst below 0.5%, the level normally adequate for a fully compounded powder unblended with natural powder, the intensity of colour is reduced.

In the course of investigations carried out with several different colours, it appears that the correct effect is achieved at about 0.5% to 0.75% or roughly 50% greater pigment content than used in a similar colour for a fully compounded powder.

In addition, care must be taken to ensure that the base resin used to make the higher strength coloured powder is similar to that of the natural powder with which it will.

be blended. If the melt viscosity and density are the same the best fusion is achieved resulting in the highest possible impact strength.

A further consideration is the ratio of coloured powder and natural powder used in the blend.

Is is an object of the present invention to provide a method of manufacturing a coloured plastics material for moulding which obviates or reduces the above disadvantages.

It is another object of the present invention to provide a tank for storage of domestic heating oil using the above coloured plastics material.

According to the present invention, there is provided a method of manufacturing a coloured plastics material formulation for moulding comprising the steps of compounding pigment with molten granules of plastics material, grinding said pigmented material to a powder form, and mixing said pigmented powder with unpigmented powder of similar plastics material.

Preferably, the amount of pigment compounded with the plastics material is in the region of 30-70%, preferably c. 50%, greater than that normally used.

In the case of most pigments, the amount of pigment should be between 0.6 to 0.75 wt.% of the component of the mix.

The preferred plastics material is polyethylene.

The mixture may ideally comprise 50-90 wt.% (preferably 75 wt.%) non-pigmented plastics powder and 10-50 wt.t (preferably 25 wt.%) pigmented plastics powder.

According to another aspect of the present invention, there is provided a coloured oil storage tank by a moulding process using a plastics material as described above.

Preferably a rotational moulding process is used.

The invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 shows a magnified view (x 25) of a thin section of moulding according to the prior'art; FIGURE 2 shows graphically the influence of colouring technique and pigment concentrations on the impact strength of polyethylene at 230C; FIGURE 3 shows graphically the influence of colouring technique and pigment concentrations on the impact strength of polyethylene at -240C; and FIGURE 4 shows the results of the UV spectrophotometer reading to show light transmission over a range of wavelengths.

It is necessary to ensure that the walls of the tank do not allow the passage of light Clearly a low level of coloured powder addition in the blend, say 10%, will not provide this important property. Depending on the colour density of the pigments used, the blend ratio will vary.

The most dense of these pigments is carbon black and the amount of carbon black present in the colour formulation will have a significant effect. If a lighter colour is required, the carbon black level will be low and a higher blend ratio is then necessary. Most domestic oil tanks are coloured green. To achieve a middle green shade the blend ratio is in the region of 75 parts of natural powder to 25 parts green powder. For a very dark green containing carbon black this ratio can be reduced, to the region of 80 parts of natural powder to 20 parts green powder. A black tank will be opaque to light at ratios as low as 9:1. However, much will depend on the size of the tank and the necessary wall thickness. For tanks in the capacity range of 200 gallons to 1000 gallons, the wall thickness averages 8mm, where the above ratios will apply.

In a series of light transmission experiments using a photospectrometer, it was found that a green composition in which the colour pigment formulation was as shown in Table I, the light transmission through an 8mm thick sample was less than 0.1% in all wavelengths from 800nm to 330 nm and was similar to the control metal plate, whereas sample of natural, unpigmented material allowed significant light transmission through all wavelengths.

(See Figure 4.) The green formulation used was a middle green colour.

Darker shades of green are commonly used and the light blocking characteristics will therefore, be even greater.

For thinner walls the coloured powder content in the blend will be higher and conversely for very large tanks with wall thicknesses of 10mm or more, the blend ratio can be reduced.

The effects of the different pigment colouring methods on impact properties when tested by a falling dart test method at +230C and -240C are shown in Figures 2 and 3.

The methods of colouring shown in Figures 2 and 3, are as follows:1. Compound Powder Pigment was added to the polyethylene during extrusion melt compounding and the coloured granules then ground to powder.

2. Low Speed Blend This is a dry blend of pigment powder and polyethylene ground powder. The blending was carried out by tumble mixing at low speed by rotating a blending drum end over end for 15 minutes.

3. High Speed Blend This is another dry blend of pigment powder and polyethylene ground powder. The blending was carried out in a high speed mixer where the blades were rotated at approximately 1000 rpm for 60 seconds.

4. Compound Blend This is a mixture of natural powder and colour compounded powder where the mix ratio was 75:25.

A falling dart impact test (ISO 6603/2) was used and the total energy to failure at various pigment concentrations was plotted.

The colouring effect achieved is better than might be expected because of the translucent nature of the natural powder component in the blend. In light colours, there is an acceptable mottled appearance and this can disappear with darker colours.

At all these levels necessary to achieve the desired colour effect, the pigment content is low enough (below 1%) not to interfere with fusion and the impact properties are retained. Levels of addition of the coloured powder component in the blend can be controlled accurately and the cost of the blend is close to the low cost dry colouring method. See Table II.

A typical pigment formulation for a mid-green tank is shown in Table I below.

TABLE I Pigments Titanium Dioxide 0.300 Phthalocyanine Blue 0.036 Lead Chrome Yellow 0.247 Carbon Black 0.001 0.584 NOTE: The powder compound pigmented as above was blended at 25% strength with natural powder.

TABLE II COMPARATIVE COSTS OF COLOURING SYSTEMS (Cost per Kilo of powdered material) Natural Powder and Unpigmented Pigmented Powdered Compounded Powder Natural Powder Dry Blend Compound Blends at 75:25 Blend Ratio (0.4%) (0.5%) Natural 0.80 0.79 0.72 0.60 COLOURING ~ 0.04 0.38 0.28 LAbour, pigment ) wastage and mould ) - 0.02 - maintenance ) 0.80 0.85 1.10 0.88 As may be seen, the method of manufacturing a coloured oil tank by rotational moulding employs a blend of natural polyethylene powder and colour compounded polyethylene powder. This does not result in any significant reduction in physical properties, but is less costly than the use of a fully compounded material.

This method of colouring an oil tank provides sufficient opacity to prevent the degradation of the oil held in it, while significantly reducing health hazards to moulding operators, does not result in pigment rich inner tank surfaces which can contaminate the tank contents and further reduce impact strength, and does not foul mould surfaces, thus reducing maintenance costs and colour change wastage.

Claims (6)

C L A I M S:

1. A method of manufacturing a coloured plastics material formulation for moulding comprising the steps of compounding pigment with molten granules of plastics material, grinding said pigmented material to a powder form, and mixing said pigmented powder with unpigmented powder of similar plastics material.

2. A method as claimed in Claim 1, wherein the amount of pigment is between 0.6 to 0.75 wt.% of the component of the mix.

3. A method as claimed in either Claim 1 or Claim 2, wherein the plastics material is polyethylene.

4. A method as claimed in any one of the preceding claims, wherein the formulation comprises 50-90 wt.% non-pigmented plastics powder and 10-50 wt.% pigmented plastics powder.

5. A method as claimed in Claim 4, wherein the formulation comprises 75 wt.% non-pigmented plastics powder and 25 wt.% pigmented plastics powder.

6. A coloured oil storage tank produced by a moulding process using a plastics material as claimed in any one of the preceding claims.