GB2288141A

GB2288141A - Chemical vessels made from carbon-pigmented HPDE

Info

Publication number: GB2288141A
Application number: GB9406947A
Authority: GB
Inventors: Cor Jansen; Peter Marriott; Joachim Leifels
Original assignee: Micro Image Technology Ltd; WIVA BV; Wiva Verpakkingen BV
Current assignee: Micro Image Technology Ltd; WIVA BV; Wiva Verpakkingen BV
Priority date: 1994-04-08
Filing date: 1994-04-08
Publication date: 1995-10-11
Also published as: GB9406947D0; WO1995027754A1

Abstract

The use of a high density polyethylene carbon pigmented composition in the manufacture by blow moulding of vessels or parts thereof suitable for storing or transporting ultra-high purity chemicals containing impurity levels of less than one part in a billion. The high density may be between 900 Kg/m<3> and 970 Kg/m<3>. A preferred carbon pigment consists of a trace element-free, pure organic molecule comprised exclusively of carbon and hydrogen and optionally oxygen atoms, for example polyolefine molecules. The pigment may be pure carbon or partially or completely oxidised hydrocarbon materials, e.g. polyolefins.

Description

CHEMICAL VESSELS The present invention relates to vessels for use in the storage and transport of ultra high purity chemicals.

Some steps in the manufacture of electronic components, such as the etching of photomasks or the cleaning of silicon surfaces of semi-conductor devices, require the use of very high purity chemicals, i.e. chemicals having an impurity level of less than one in a billion of a range of 30-40 individual trace metals such as iron or calcium.

Examples of commonly used high purity chemicals include hydrofluoric acid and sulphuric acid.

Problems have arisen in transport of the ultra pure chemicals from the site of manufacture to the site of use and in storage thereafter. It is important that the chemical maintains its purity and that foreign chemicals do not leach out from the material of the transport and storage vessel. For example, certain acids, e.g. hydrofluoric acid, can diffuse into the walls of the vessel causing leaching and as a result can become contaminated by the leached impurities from the vessel.

The present state of the art uses vessels made of polyfluoroalkoxy (PFA) materials. These materials have very low inherent leachable levels. For example, unlike other polymers, there is a very low residual level of catalyst within the material. Such catalysts are prone to leakage from the polymer. PFA vessels are generally made by roto moulding of granular material. Granules are fed into the mould, which is heated. The mould is then spun.

As the PFA granules heat up, they form a flowable liquid which, due to the spinning force, coats the inside of the mould. The mould is allowed to cool and the finished vessel is removed. This process is slow and labour intensive, and therefore results in expensive products.

Furthermore it is difficult to prevent contamination occuring during the process.

Other moulding processes have been proposed, but the mechanical and chemical properties of PFA which make it suitable for transporting certain chemicals make it difficult to mould by other methods. For example, it is very difficult to incorporate mould release agents into the material. It is also difficult to incorporate materials which can strengthen the vessel against breakage, e.g. impact modifiers. As safety regulations become more strict, it is necessary that vessels become stronger, and if the polymeric material cannot be strengthened, this requires thicker walling which leads to increased manufacturing costs and increased vessel weight.

Blow moulding can only be used in the production of vessels up to 5 litre capacity, due to the mechanical properties of PFA.

It is known that some materials are transportable in polyethylene vessels. The material is supplied as granules or powder for use in large blow and injection moulded applications. The polyethylene is cheaper than PFA and has a high impact strength and high stress cracking resistance. Materials formed are highly rigid and have very good chemical compatibility. Blow moulding is a much cheaper method of manufacture than roto moulding.

Low density polyethylenes, whilst suitable for transporting many liquids, are not acid-resistant.

Therefore it is necessary to manufacture containers from high density polyethylene (HDPE). Two processes are used in the manufacture of such high density products, the Ziegler process, which requires high catalyst levels, or the Philips process, which requires low catalyst levels.

In order to produce containers which will not leach catalysts into the contained high purity liquid, it is necessary to use the Philips process.

Polyethylene is a transparent material. This can cause problems in the transport of certain materials since light can lead to breakdown of certain liquids, for example hydrogen peroxide, and the polyethylene structure itself.

In order to overcome this problem, it has been found that opacifying agents such as titanium dioxide or calcium carbonate can be added to the polyethylene. This darkens the polymeric material sufficiently to stop transmission of light levels which break down the container contents and also stabilises the polyethylene. A disadvantage of this type of pigment is that additional contaminants such as titanium or calcium are introduced via the pigment and can lead to contamination of the chemicals stored in such pigmented vessels.

The present Applicants have now discovered that selected grades of high density < -polyethylenes containing special carbon pigments are suitable for the production of vessels by blow moulding for the transport of ultra high purity chemicals. The vessels are cheaper, stronger and have lower inherent contaminants than those formed using conventional PFA materials. Furthermore, the vessels according to the invention do not suffer from pigment attack by aggressive acids.

Accordingly, the present invention relates to the use of a high density polyethylene carbon-pigmented composition in the manufacture by blow moulding of vessels or parts thereof suitable for storing or transporting ultra-high purity chemicals containing impurity levels of less than one part in a billion.

Preferably substantially no impurities are leached into the chemical over a period of at least six months.

An example of a suitable high density polyethylene has a density of between 900 Kg/m3 and 970 Kg/m3 (BS 3412 1976 ISO 1872 - 1972) and a melt flow rate, under a load of 21.6 Kg at a temperature of 190"C of 1.5 g to 2.5 g in 10 minutes (BS 3412 1976 - ISO 1133 - 1981 Condition 7).

This is a high molecular weight homopolymer having very high impact strength' and stress crack resistance characteristics. A material is supplied as powder or granules which can be mixed with pigment and used in blow moulding processes.

A preferred carbon pigment consists of a trace elementfree pure organic molecule comprised exclusively of carbon and hydrogen and optionally oxygen, atoms, for example polyolefine molecules, being partially or completely oxidised under clean controlled conditions and being capable of absorbing electromagnetic radiation preferably in the range of 10 nanometers to 10 micrometers and more preferably in the range of 500 nm to 50 nm.

Preferably the pigment is essentially pure carbon.

Optionally the pigment comprises partially or completely oxidised hydrocarbon materials such as polyolefins.

A preferred pigment is a carbon black pigment with a pigment size of 0.5 ym to 200 ym, more preferably in the range of 5 ym - 20 ym which is trace element-free.

Preferably the trace element content is less than 1 ppm in the pigment.

As these pigments have to be added to the high density polyethylene granules, the pigment can preferably be encapsulated in high -density polyethylene granules or powder called hereafter pigment granules. These pigment granules have a preferred density between 900 Kg/m3 and 1200 Kg/m3 (BS 3412, 1976 - ISO 1872, 1972) and a melt flow rate under a load of 5 Kg at a temperature of 1900C of preferably 3 g to 12 g in 10 minutes (ISO 1133, 1981, condition 5). The corresponding amount of pigment in the pigment granules is preferably between 20% and 80% by weight, and more preferably between 40% and 60% by weight.

By virtue of their molecular structure these pigments do not contribute significantly to contamination and are substantially resistant against high purity acidic or alkaline solutions.

Preferably, the pigment is added to the polyethylene in a range of 0.01% to 10%, and more preferably in the range 0.2 to 4.0% by weight.

The combination of the described high density polyethylene with the addition of the described carbon pigment can be characterised by the leachability of metal ions from vessels by measurement of iron and calcium, levels leached into high purity 50% hydrofluoric acid itself having a contamination level of less than one part per billion.

The maximum levels of each of these elements which may be present are one part per billion after an extraction time of six months or less at ambient storage temperature.

Examples of chemicals which may be transported in the containers are hydrochloric acid, hydrofluoric acid, nitric acid, hydrogen peroxide, ammonium hydroxide, mixtures of hydrofluoric acid and ammonium fluoride (buffered oxide etches), sulphuric acid, phosphoric acid, potassium hydroxide, tetramethylammonium hydroxide, and certain solvents such as isopropyl alcohol and acetone.

By way of an example illustrating the invention, a 200 L drum has been manufactured using a high density polyethylene as described above with the addition of carbon black pigment also described above. The addition of the pigment to the polyethylene granules was chosen as 2.5% by weight. The blow moulding process temperature is about 200or and therefore, in order to protect the polyethylene against oxidation, a standard phenyl based anti-oxidant was also added. The added amount of the anti-oxidant was chosen to be 0.1% by weight. For the blow moulding process filtered air was used with a final filtration at 0.2 ym at a pressure of 1 MPa. In order to avoid contamination during the cooling phase after the blow moulding process, the drums have been stored in a clean air environment.

Table 1 shows the evolution over time of the contamination levels in NH4OH solution in the drums manufactured as above. The drums were stored at ambient temperature and measured every four weeks after filling. The results expressed in parts per billion [ppb] show that within the accuracy of our measurement, the pollution contributed by the drum is less than 0.1 ppb for each individual trace element. The measurement of trace element was done using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Atomic Absorption Spectrophotometry (AAS) for the elements Fe, Ca, Na and K. The detection limit for Fe, Ca, Na and K is 0.01 ppb with an accuracy of + or - 0.005 ppb.

Similar tests have been done with chemicals like HNO3, H2SO4, H2O2, HC1 and HF showing that the drums maintain a purity level of less than one part per billion for each individual trace element.

LAPORTE ELECTRONIC CHEMICALS MBPATENTNH.XLS TABLE 1 LEACHING OF CONTAMINANTS FROM A ZOO L H.D.P.E. DRUM INTO NH4OH SOLUTION RESULTS on ppb

RAW DRUM AFTER AFTER AFTER AFTER MATERIAL FILLING 4 WEEKS 8 WEEKS 12 WEEKS STORAGE STORAGE STORAGE DATE 22/12/1993 23/12/1993 17/01/1194 17/02/1994 18/03/1994 ASSAT 29.3 29.3 29.3 29.3 29.3 PO4 3- < 500 < 500 < 500 < 500 < 500 SO4 2- < 2000 < 2000 < 2000 < 2000 < 2000 CL- < 500 < 500 < 500 < 500 < 500 Al < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 As < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Sb < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Ba < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Bi < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 B < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Ca 0.18 0.18 0.17 0.18 0.19 Cd < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Co < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Cu < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 C1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Ga < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Ge < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Au < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Fe 0.05 0.05 0.06 0.07 0.09 Pb < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Li < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Mg < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Mn < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Mo < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Ni < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 K 0.03 0.04 0.06 0.05 0.05 P < 50 < 50 < 50 < 50 < 50 Si < 50 < 50 < 50 < 50 < 50 Ag < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Na 0.08 0.07 0.08 0.08 0.09 Sr < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Sn < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Ti < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 V < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Zn < 0.1 < 0.1 < 0.1 < 0.1 < 0.1

Claims

CLAIMS 1. The use of a high density polyethylene carbon pigmented composition in the manufacture by blow moulding of vessels or parts thereof suitable for storing or transporting ultra-high purity chemicals containing impurity levels of less than one part in a billion.
2. The use as claimed in Claim 1 wherein substantially no impurities are leached into the stored chemical from the vessel over a period of at least 6 months.
3. The use as claimed in Claim 1 or 2 wherein the carbon pigment consists of a trace element-free, pure organic molecule comprised exclusively of carbon and hydrogen and optionally oxygen atoms.
4. The use as claimed in Claim 1, 2 or 3 wherein the carbon pigment comprises partially or completely oxidised polyolefin molecules.
5. The use as claimed in any one of the preceding Claims wherein the carbon pigment is capable of absorbing electromagnetic radiation in the range of 10 nanometers to 10 micrometers.
6. The use as claimed in any one of the preceding Claims wherein the carbon pigment is a carbon black pigment having a pigment particle size in the range of 5 - 20 ym.
7. The use as claimed in any one of the preceding Claims wherein the pigment is encapsulated in high density polyethylene granules or powder.
8. The use as claimed in Claim 7 wherein the pigment granules have a density in the range of 900 Kg/m3 to 1200 Kg/m3.
9. The use as claimed in Claims 7 or 8 wherein the pigment granules have a melt flow rate under a load of 5 Kg at a temperature of 1900C of 3 gms - 12 gms in 10 minutes.
10. The use as claimed in any one of the preceding Claims wherein the pigment is added to the polyethylene in the range of 0.01% to 10% by weight.