WO1994016045A1

WO1994016045A1 - Machine dishwashing composition containing a corrosion inhibitor

Info

Publication number: WO1994016045A1
Application number: PCT/EP1994/000025
Authority: WO
Inventors: Petrus Adrianus J. M. Angevaare; Frederick Hornung
Original assignee: Unilever NV
Current assignee: Unilever NV
Priority date: 1993-01-18
Filing date: 1994-01-06
Publication date: 1994-07-21
Anticipated expiration: 1995-07-18
Also published as: AU5860194A

Abstract

In a machine diswahsing operation the risk of metal corrosion, especially of stainless steel to pitting corrosion, is significantly reduced by using a compound of formula (I): HOOC-(CH2)m-(CX1X2)n-(CH2)y-COOH, wherein m is from 0-1, n is from 0-2 and y is from 0-1; and X1 and X2 represent each individually H, OH, NH2, COOH, C1-C6 alkyl or alkoxy group, NR2 or COOR, wherein R is C1-C6 alkyl or the water-soluble salts thereof, with the proviso that if m, n and y are 1, then CX1X2 is not C(OH) (COOH). The corrosion inhibiting compound may be added as ingredient of a machine dishwashing composition or as ingredient of a rinse aid composition.

Description

MACHINE DISHWASHING COMPOSITION CONTAINING A CORROSION INHIBITOR

This invention relates to machine dishwashing and mechanical warewashing which for briefness' sake will be further referred to as machine dishwashing.

Usually, in machine dishwashing operations, the articles, such as pots, pans, dishes, cups, saucers, bottles, glassware, crockery, cutlery and other kitchen utensils, to be cleaned are first cleansed in a main wash step using a main wash product (machine dishwashing composition) and subsequently rinsed several times to rinse away any adhering main wash product and remaining soil. It is customary to add to the rinsing water in the rinse step a rinse aid composition which causes the articles to dry more evenly and improves the visual appearance of the articles when dry.

A generally regarded problem of machine dishwashing, however, is the susceptibility of metals, especially stainless steel, to pitting corrosion, i.e. stainless steel articles, such as spoons, forks and cutlery, washed in a dishwashing machine are susceptible to pitting corrosion. Though various factors can be indicated that induce pitting, chloride concentration is generally regarded as the most important factor. It is believed that after the final rinse is finished, water droplets can remain on articles, e.g. knives. During the drying stage, the chloride concentration in the water droplets will gradually increase, due to evaporation of the water. The stainless steel cutlery is subsequently subjected to both high temperature (60-70°C) and humidity. The initial pits formed during the final rinse can then grow rapidly to give visible rust.

Conventional machine dishwashing compositions are highly alkaline, phosphate-built products comprising a chlorine bleach. In the last few years, a new range of machine dishwashing products has come to the market. These are enzymatic products which are less alkaline and in general comprise a non-phosphate builder and a peroxygen bleach.

Typically, it has been observed that the pitting problem is more serious with the lower pH-type machine dishwashing compositions than with the more conventional, highly alkaline-type machine dishwashing products.

It is therefore an object of the present invention to provide means for minimising the risk of stainless steel pitting corrosion in machine dishwashing when using a wash liquor at a pH in the range of 8-11.

It is another object of the invention to provide an improved machine dishwashing composition with a reduced risk of causing pitting corrosion to stainless steel articles washed therewith, at a pH in the range of 8-11.

Still another object of the invention is to provide a rinse aid composition for use in an aqueous rinse liquor in the rinse step of a machine dishwashing operation to efficiently inhibit stainless steel pitting corrosion.

These and other objects, as will be apparent from the description hereinafter, can be accomplished according to the invention by the use of a compound of formula I below:

HOOC-(CH₂)_m-(CX₁X₂)_n-(CH₂)_y-COOH (I)

wherein m is from 0-1, n is from 0-2 and y is from 0-1; and X_λ and X₂ represent each individually H, OH, NH₂, COOH, _l~^c ₆ ^alkyl ^or alkoxy group, NR₂ or COOR, wherein R is C_j-C₈ alkyl or the water-soluble salts thereof, with the proviso that if , n and y are 1, then CX^^ is not C(0H) (COOH) , as a stainless steel pitting corrosion- inhibiting ingredient of a machine dishwashing composition or rinse aid composition, which are formulated such that they provide a wash liquor with a pH of 8-11 (measured at a concentration of 0.1-0.3% by weight in water) .

Preferred compounds of formula I are those wherein n = 1, preferably wherein n = 1 and m = 0, more preferably wherein n = 1, m = 0 and y = 0.

Preferably, X is H and X₂ is H, OH, NH₂ or a ^-C₈ alkyl group; preferred alkyl groups are 0 -0 alkyl, i.e. methyl, ethyl, propyl and butyl groups.

Examples of suitable and preferred compounds within the invention are oxalic acid, alonic acid, succinic acid, glutaric acid and derivatives thereof, such as hydroxy malonic acid, methyl, ethyl or butyl malonic acid, propane- 1,2,3-tricarboxylic acid, and aspartic acid.

Especially preferred are malonic acid and derivatives thereof.

Accordingly, the first subject of the invention is the novel use of a compound of formula I above as corrosion inhibitor in a machine dishwashing operation. The corrosion inhibiting compound according to the invention may be added, as ingredient of a machine dishwashing composition, to the rinse water as such, or as ingredient of a rinse aid composition.

Therefore, in one broad aspect the invention provides a process for machine dishwashing comprising a wash cycle and a rinse cycle, characterised in that a compound of formula I as hereinbefore described is added to the wash liquor and/or to the rinse liquor. In a more specific aspect, the invention provides a machine dishwashing composition, characterised in that it comprises a compound of formula I as hereinbefore defined.

In another specific aspect, the invention provides a rinse aid composition for use in an aqueous rinse liquor in the rinse step of a machine dishwashing operation, characterised in that said composition comprises a compound of formula I as hereinbefore defined.

The corrosion inhibiting compounds as hereinbefore defined are biodegradable and can be used in the compositions according to the invention at any level ranging from about 0.1% to about 20% by weight.

Within these ranges, it applies as a rule that when the compound is delivered in the wash through the main wash product, the amount required is generally higher than when the compound is added as rinse aid in the final rinse. An obvious explanation thereof is that pitting inhibition through a main wash product is mainly achieved by carry¬ over to the final rinse.

Preferred levels are therefore from about 2 to 15% by weight for machine dishwashing compositions, and from about 0.5 to 5% by weight for rinse aid compositions, particularly from about 3 to 10% and from about 1 to 3%, respectively.

Without wishing to be bound by any theory, it appears that there is a relationship between corrosion and pitting (initiation) potential of the metal surface, (E_pit) , which is the electrochemical potential (in mV) at which the protective film (on the metal surface) starts to break down. Hence E _it can be used to characterise the corrosiveness of a medium and the susceptibility of metals to pitting by measuring the E_pit. The higher the E _it, the higher the corrosion resistance.

The machine dishwashing composition of the invention will normally also contain a builder or builder mixture, optionally also buffering and/or alkaline agents, and preferably also a bleaching agent and an enzyme or mixtures of enzymes.

The builder material Builder materials (phosphates and non-phosphate builder materials) are well known in the art and many types of organic and inorganic compounds have been described in literature. They are normally used in all sorts of cleaning compositions to provide alkalinity and buffering capacity, prevent flocculation, maintain ionic strength, extract metals from soils and/or remove alkaline earth metal ions from washing solutions.

The builder material usable herein can be any one or mixture of the various phosphate and non-phosphate builder materials. Preferred builder materials are non-phosphate builders such as, for example, the alkali metal citrates, carbonates and bicarbonates; nitrilotriacetic acid (NTA) ; dipicolinic acid (DPA) ; oxydisuccinic acid (ODS) ; alkyl and alkenylsuccinates (AKS) ; zeolites; layered silicas and mixtures thereof. They may be present in the composition of the invention in an amount of from 10% up to about 90% by weight, preferably from 20% to 80% by weight.

Particularly preferred builders are citrates, DPA, ODS, alkenylsuccinates, carbonates, bicarbonates, zeolite, and block copolymers ITA/VA as described in Applicant's co- pending GB application N° 9210869.5.

The buffering and/or alkaline agent

This is normally an alkali metal silicate, preferably sodium silicate at a level of from about 1 to about 70% by weight, preferably from 5 to 40% by weight. This material is employed as a cleaning ingredient, source of alkalinity, metal corrosion inhibitor and protector of glaze on china tableware. Especially effective is sodium silicate having a mole ratio of SiO₂:Na₂0 of from about 1.0 to about 3.3, preferably from about 1.8 to about 2.2, normally referred to as sodium disilicate.

NaOH and/or KOH are also commonly used to provide alkalinity in compositions for industrial warewashing machines.

The bleaching agent

If present, the amount of bleaching agent will preferably lie in a range from 1 to 30% by weight. Alkali metal hypochlorite may be incorporated. Other chlorine bleaches are alkali metal salts of di- and tri-chloro and di- and tri-bromo cyanuric acids.

Preferred bleaches are the peroxygen bleaches, such as sodium perborate (tetra- or monohydrate) or sodium percarbonate. These are preferably used in conjunction with a bleach activator which allows the liberation of active oxygen species at a lower temperature. Numerous examples of activators of this type, often also referred to as bleach or peracid precursors, are known in the art. Preferred bleach activators are tetraacetyl ethylene diamine (TAED) , glucose pentaacetate (GPA) and the mono long-chain acyl tetraacetyl glucoses as disclosed in WO 91/10719, but other activators such as choline sulphophenyl carbonate (CSPC) as disclosed in US Patents 4,751,015 and 4,818,426 can be used. The amounts of sodium perborate or percarbonate and bleach activator in the compositions preferably do not exceed 20% and 10% by weight, respectively, e.g. from 4-20% and from 2-10% by weight, respectively. Another peroxygen bleach is potassium monopersulphate. Further peroxygen bleaches which may be used are the organic peroxyacids and their metal salts. Typical peroxyacids include: (i) peroxybenzoic acid and ring-substituted peroxybenzoic acids, e.g. peroxy- -naphthoic acid;

(ii) aliphatic and substituted aliphatic monoperoxy acids, e.g. peroxylauric acid and peroxystearic acid; (iii) 1, 12-diperoxydodecanedioic acid (DPDA) ; (iv) 1,9-diperoxyazelaic acid;

(v) diperoxybrassylic acid; diperoxysebacic acid and diperoxyisophthalic acid;

(vi) 2-decyldiperoxybutane-l,4-dioic acid; and (vii) phthaloylamido peroxycaproic acid (PAP) .

Instead of, or together with said bleach activators, a bleach catalyst, such as the manganese complexes of EP-A- 458,397 and the sulphonimines of US Patents 5,041,232 and 5,047,163 may also be added.

Enzymes

Amylolytic and/or proteolytic enzymes are normally and preferably used in the composition of the invention. The amylolytic enzymes usable herein can be those derived from bacteria or fungi. Preferred amylolytic enzymes are those prepared and described in GB Patent N° 1,296,839, cultivated from the strains of Bacillus licheniformis NCIB 8061, NCIB 8059, ATCC 6334, ATCC 6598, ATCC 11945, ATCC 8480 and ATCC 9945 A. Examples of such amylolytic enzymes are those produced and distributed under the trade-names of SP-95® and Termamyl® by Novo Industri A/S, Copenhagen, Denmark. These amylolytic enzymes are generally presented as granules and may have enzyme activities of from about 2 to 10 maltose units/milligram. They may be present in the composition of the invention in amounts such that the final composition has amylolytic enzyme activity of from 10³ to 10⁶ maltose units/kilogram. The amylolytic activity as referred to herein can be determined by the method as described by P. Bernfeld in "Method of Enzymology", Volume I (1955), page 149.

The proteolytic enzymes usable herein are, for example, the subtilisins which are obtained from particular strains of β. Subtiliε and B . licheniformis , such as the commercially available subtilisins Maxatase® supplied by Gist-Brocades NV, Delft, Holland, and Alcalase®, supplied by NOVO Industri A/S, Copenhagen, Denmark. Particularly suitable are proteases obtained from a strain of Bacillus having maximum activity throughout the pH range of 8-12, being commercially available from NOVO Industri A/S under the trade-names of Esperase® and Savinase®. The preparation of these and analogue enzymes is described in GB Patent N° 1,243,784. These enzymes are generally presented as granules, e.g. marumes, prills, T-granulates etc., and may have enzyme activities of from 500 to 1700 Glycine Units/mg. The proteolytic enzyme activity can be determined by the method as described by N.L. Anson in "Journal of General Physiology", Vol. 22 (1938), page 79 (one Anson unit/gram = 733 Glycine Units/milligram) .

In the compositions of the invention, proteolytic enzymes may be present in amounts such that the final composition has proteolytic enzyme activity of from about 10⁶ to 10⁸ Glycine Units/kilogram.

Other enzymes, such as lipolytic enzymes, may also be incorporated to improve fat removal. Typical examples of commercial lipolytic enzymes are Lipase YL, Amano CE, Wallerstein AW, Lipase My, and Lipolase ex Novo Ind.

Other optional ingredients Surfactants, though not strictly essential, may also be present for detergency, soil removal, foam depression and/or as rinse aids. If present, they can be used in an amount of up to about 60% by weight, depending upon their type and properties. Normally in a properly built or highly built composition as is conventional, only small amounts of low- to non-foaming nonionic surfactant in the order of 0.1-5% by weight are used, preferably from 0.1-4%, to aid detergency and particularly to suppress excessive foaming caused by some protein soil. Higher amounts, i.e. 5-60% by weight of highly detersive surfactants, such as the high HLB nonionic surfactants, the anionic sulphate or sulphonate surfactants and the alkyl polyglycoside class of surfactants, may be used in lower builder containing active/enzyme-based compositions. In the context of the present invention, a surfactant content from 0 to less than 5% by weight is preferred.

Other optional ingredients that can be further included in minor amounts are clay minerals, particularly the layered clay minerals to reduce film and spot formation on washed articles. Typical and particularly preferred commercial clay products are the synthetic hectorites manufactured and supplied by Laporte Industries Ltd, England as Laponite® clays, e.g. Laponite S, Laponite XLS, Laponite RD and Laponite RDS. Zinc salts, both soluble and insoluble zinc salts, can also be incorporated as adjuncts for minimizing glass corrosion. Furthermore, anti-sealants, such as the polyphosphonates and polypeptides, may be present.

An inert particulate filler, especially sodium sulphate, may also be incorporated, though in compact powdered composition it should desirably be omitted as practically possible.

The products of the present invention can be manufactured and presented in any physical form such as a powder, liquid, paste or solid blocks or tablets, and are formulated such that they provide a wash liquor with a pH of from 8-11, preferably from 9.0-10.5 (measured at a concentration of from 0.1-0.3% by weight in water) .

As explained, the invention is also applicable to rinse aid compositions. They are normally presented as an aqueous solution comprising a low-foaming nonionic surfactant, citric acid or sodium citrate and optionally a hydrotrope such as a lower alcohol or sodium xylene sulphonate. For the purpose of the invention, the rinse aid composition may be a simple aqueous solution comprising the corrosion inhibiting compound.

The invention will now be further illustrated by way of the following non-limiting Examples.

Examples I and II

Model pitting potential measurements

Corrosion inhibitor efficiency was judged on stainless steel knives exposed to chloride-containing solutions by electrochemical testing. Novinox® knives, which were found to be extremely corrosion-sensitive, were used as model substrates. Before use, the knives were degreased/cleaned by thorough washing with a hand dishwash liquid detergent, followed by rinsing with acetone.

Pitting potentials (E _it) were measured by the potentiodynamic polarization method - with a scan rate of 0.2 mV/s - in an aerated H-cell with a platinum counter electrode (CE) . The surface area of the Pt-plate immersed in the solution was about 9 cm². The working electrode (WE) was the stainless steel Novinox® knife (the surface area of the knife dipped in the solution may not be more than 2 cm²) and a saturated calomel electrode (SCE) as reference electrode. An EG&G Princeton Applied Research Model 384B Polarographic Analyzer was used to obtain the polarization curves registered by a Bausch & Lomb DMP-40 series digital plotter or by a PC taken in a Lotus file. Unless otherwise mentioned, the measurements were carried out at 70°C and the solution in the "working electrode room" was agitated smoothly by means of a magnetic stirrer. In order to effect good electrical conductivity, sodium acetate (0.2 M) was used as supporting electrolyte. This salt was chosen because it did not interfere with the measurement.

The solution used was a simulated main wash product carry¬ over to final rinse (pH 8.2) consisting of 3 mg/1 of a product having the following composition (parts by weight) :

Sodium citrate 43.0

Sodium disilicate 34.0

Sodium perborate monohydrate 6.8 TAED 4.2

Savinase proteolytic enzyme 1.7

Termamyl amylolytic enzyme 1.7

Laponite clay 1.7

Nonionic surfactant 1.7

to which there were added 1 g/1 NaCI and 0.2 M sodium acetate, and a compound X.

The influence on the pitting potential of compound X at 20, 50 and 80 μmol/1 is as shown in the following Tables 1 and 2.

Table l¹ ^Ep +• fmV)

Compound X 20 umol/1 50 umol/1 80 umol/1

— 300 300 300

Oxalic acid 370 450 510

Malonic acid 580 650 660

Succinic acid 430 480 530

Glutaric acid 350 400 450

Propane-1, 2,3- 350 400 450 tricarboxylic acid

L-aspartic acid 550 600 650 The following E_pit values were measured, using an EG&G Princeton Applied Research Model 273A Potentiostat/ Galvanostat with a three-electrode system SCE, CE and WE.

¹ The standard deviation for these experiments is about 30 mV.

The pitting potentials as derived from the polarization curves were performed, using an EG&G PARC Model 352

SoftCorr ^t II as installed on a Compaq 386 S/20 PC which controls the potentiostat/galvanostat.

Table 2²

-^Epi (mV)

Compound X 20 ' umol/1 50 mol/1 80 umol/1

— 270 270 270

Malonic acid 310 385 430

HydroxymaIonic acid 310 350 420

Methyl malonic acid 340 430 480

Betyl malonic acid 350 460 525

Examples III and IV

Experiments were made under machine conditions to examine the pitting corrosion inhibiting effect of malonic acid as dosed in the main wash product (III) and as dosed in the final rinse (IV) . A European dishwashing machine, Bauknecht® GS-870S (8.8 litres), was used with a programme comprising a 65°C main wash and final rinse at 65°C.

The main wash product formulation was the same as that used in Examples I and II and dosed at 3 g/1; for each run 1 g/1 NaCl was dosed in the final rinse to create an artificially severe corrosive condition for obtaining accelerated results. The machine contained standard load and 2 or 3 Novinox® stainless steel knives kept apart from each other in the cutlery basket. In accordance with the recommendations of CATRA*, the door of the machine was kept closed at the end of each wash cycle and allowed to stand overnight (about 20 hours) . The knives were then analyzed for the presence of corrosion pits.

* CATRA = Cutlery & Allied Trade Research Associations, Sheffield, U.K.

² The standard deviation for these experiments is about 30 mV

In the Example III experiments, the percentage by weight of malonic acid added to the main wash product was set out against the number of runs, after which corrosion pits appeared. The results were as follows:

Malonic acid (% by weight) Number of runs

0 1 1 1

3 1

5 2

10 4

15 4

In the example IV experiments, the concentration of malonic acid in the final rinse (mg/1) was set out against the number of runs, after which corrosion pits appeared. The results were as follows:

Malonic acid (mg/D* Number of runs

0 1

2 4

10 6

* 1 mg/1 corresponds to 0.5% by weight in a rinse aid composition used at a dosage of 1 ml/5 litres.

Claims

1. Use of a compound of formula :

HOOC-(CH₂)_m-(CX₁X₂)_n-(CH₂)_y-COOH (I)

wherein m is from 0-1, n is from 0-2 and y is from 0-1; and X_χ and X₂ represent each individually H, OH, NH₂, COOH, _l~ ₆ alkyl or alkoxy group, NR₂ or COOR, wherein R is C-_j^-C₈ alkyl or the water-soluble salts thereof, with the proviso that if m, n and y are 1, then CX_χX₂ is not C(OH) (COOH) , as a stainless steel pitting corrosion- inhibiting ingredient of a machine dishwashing composition or rinse aid composition, said composition having a solution pH of from 8-11 (measured at a concentration of 0.1-0.3% by weight in water).

2. Use according to Claim 1, characterised in that n = 1.

3. Use according to Claim 2, characterised in that m = 0 and y = 0.

4. Use according to any of Claims 1-3, characterised in that X_x is H, and X₂ is H, OH, NH₂ or a C-^C_g alkyl group.

5. Use according to Claim 4, characterised in that the compound is selected from oxalic acid, malonic acid, succinic acid, glutaric acid, hydroxy malonic acid, C-^^ alkyl malonic acid, propane-1,2, 3-tricarboxylic acid and aspartic acid.

6. A machine dishwashing composition or rinse aid composition comprising a builder or builder mixture, characterised in that it further comprises from about 0.1 to 20% by weight of a compound of formula : HOOC- ( CH₂ ) _m- ( CX₁X₂ ) _n- ( CH₂ ) _y-COOH ( I )

wherein m is from 0-1, n is from 0-2 and y is from 0-1; and X₂ and X₂ represent each individually H, OH, NH₂, COOH, C₁-C₆ alkyl or alkoxy group, NR₂ or COOR, wherein R is C-^C_g alkyl, or the water-soluble salts thereof, with the proviso that if m, n and y are 1, then CX₂X₂ is not C(OH) (COOH) , as a stainless steel pitting corrosion-inhibiting ingredient, said compositions having a solution pH of from 8-11 (measured at a concentration of 0.1-0.3% by weight in water) .

7. A composition according to Claim 6, characterised in that it further comprises a peroxygen bleach.

8. A composition according to Claim 6 or 7, characterised in that it further comprises an enzyme.

9. A composition according to Claim 8, characterised in that said enzyme is selected from amylolytic enzymes and proteolytic enzymes and mixtures thereof.

10. A composition according to any one of the aforementioned Claims 6-9, characterised in that the composition has a solution pH (measured at a concentration of from 0.1 to 0.3% by weight in water) of from 9.0 to 10.5.