Sodium percarbonate is traditionally made in a crystallization process from aqueous hydrogen peroxide and sodium carbonate with recycle of mother liquor in the presence of a substantial concentration of a salting out agent, conventionally sodium chloride. The invention provides a process intended to be conducted without addition of chloride salting-out agent in which the dissolution step is controlled to restrict the sodium carbonate concentration to below 95%, preferably 60 to 90% of its saturated concentration at a temperature preferably controlled to at most 35C. and the mole ratio of hydrogen peroxide:sodium carbonate in the reaction vessel is controlled to between 0.7 to 1.2:1, and preferably about 1:1. In some embodiments at least a fraction of the mother liquor introduced into the dissolution tank is obtained by extraction from the reaction vessel and in other embodiments a fraction of the sodium percarbonate is introduced in solution in the mother liquor and a fraction is introduced in particulate form directly into the reaction vessel. Large particle sodium percarbonate is provided which has a 7 day aged heat emission at 40C. of not more than 3 xcexcW/g measured in a microcalorimeter over 16 hours that is lower than the heat emission from conventionally produced sodium percarbonate employing a chloride salting-out agent.

This invention relates to a process for producing a persalt and more particularly to the production of sodium percarbonate and sodium percarbonate so produced. It also relates to sodium percarbonate itself, and to compositions containing it.
Sodium percarbonate as used herein is the name commonly employed in industry for sodium carbonate peroxyhydrate having the formula Na2CO3.1.5H2O2 and is often referred to as PCS. Although PCS has a wide range of uses employing its oxidant and disinfectant qualities, its currently most widespread use is as a bleach in washing compositions and especially the compact and super-compact compositions that have been promoted by the detergent industry since the 1980s, or in dish washing compositions or as a bleach additive or in hard-surface cleansing compositions.
Sodium percarbonate can be produced by a variety of process routes, normally employing a reaction between hydrogen peroxide and a source of sodium and carbonate. In some variations, often called collectively as xe2x80x9cwet processesxe2x80x9d, the process comprises forming an aqueous solution of sodium carbonate, mixing it with an aqueous solution of hydrogen peroxide under reaction conditions including the concentrations of the sodium carbonate and hydrogen peroxide and temperature selected such that sodium percarbonate (PCS) crystallises out of solution. The solid PCS product is then separated from the solution, often by filtration or centrifugation, is possibly subjected to further surface treatments or granulation, and dried. The separated solution, which is usually referred to as mother liquor, is discarded, forms the basis for a surface treatment or is recycled. Although a wet process could be carried out in a batch-wise fashion, it is usually carried out continuously.
In order to recover a greater fraction of product from the solution in a wet process, the practice has been commonly adopted for many years of conducting the reaction in the presence of an otherwise unreactive sodium salt such as specifically sodium chloride or adding the salt to promote further crystallisation before the solid PCS is separated. This practice, typically referred to as xe2x80x9csalting outxe2x80x9d, also drives the reaction further towards completion, converting a higher fraction of the reactants into the desired product and as a further consequence enables greater production to be obtained from a reactor of a given size. The mother liquor after separation from solid PCS contains residual concentrations of sodium and carbonate ions and hydrogen peroxide, and it will be recognised that these concentrations would be lowered as a result of the presence or introduction of the salting-out agent.
Like any process that it is desired to operate commercially, it is desirable, in principle, to minimise reagent losses, for example by recycling at least a fraction of the mother liquor to dissolve further sodium carbonate in a make-up tank before transfer into the reaction vessel. This has the additional advantage of avoiding or minimising the proportion of mother liquor which is discharged into the environment. With increasingly stringent controls being introduced in many countries on what can legally be discharged into water courses or the aquifer, the cost has increased of pre-treatment of alkaline waste liquors like mother liquors before discharge. However, there are inherent practical difficulties associated with recycling mother liquor. In particular, because it contains residual hydrogen peroxide, there is risk that on introduction of sodium carbonate into the make-up tank, further crystallisation of sodium percarbonate will be induced, producing a dispersion of solid fines that subsequently act as seeds in the reactor, so that the resultant product has less desired properties. In particular, a product which thereby contains a high proportion of small particles is rendered significantly less desirable for incorporation in compact and super-compact washing compositions. Ausimont spa in EP-A-748764 has drawn attention to the problem of premature crystallisation and solves the problem by treating the mother liquor before it is recycled to the soda ash dissolution vessel to deliberately decompose hydrogen peroxide. However, such a solution inevitably impairs process economics by increasing the waste of an important reactant.
Even if the hydrogen peroxide is not decomposed as a deliberate action in the mother liquor, there is a potential for its decomposition during the step of dissolution of sodium carbonate into recycled mother liquor, particularly where conditions are chosen to promote the rate and extent of dissolution, such as the elevated temperatures commonly employed in current continuous processes that employ sodium chloride as salting-out agent. The extent of loss of hydrogen peroxide under otherwise identical conditions in the dissolution tank is naturally related to its concentration in the mother liquor and consequently the problem would be exacerbated if little or no salting out agent is employed in the crystalliser, for the reasons identified above.
The use of a salting-out agent remains common practice in wet processes, in view of the significant advantages identified above. However, its presence in the precipitated PCS reduces the content of hydrogen peroxide which is available for release from the solid. Additionally and importantly, it has been suggested that the most effective salting out agent and the one which has been employed commercially, sodium chloride, can adversely affect the stability of PCS, but there is contrary teaching on this point. For example, Kemira in WO 9405594 has suggested that the presence of sodium chloride actually improves the stability of PCS.
Although, in theory, a wet process can be operated without addition of a salting-out agent, the absence of the salting out agent results in a lower conversion of reagents to PCS and a lower recovery of PCS from the liquor, so that the recycled mother liquor contains a higher concentration of hydrogen peroxide, thereby increasing the likelihood of premature crystallisation on recycle as well as the extent of hydrogen peroxide losses during recycle.
Expressed most starkly, the problem facing a persalt manufacturer who might wish to use a wet process to make PCS is that the wet process in common practice employs a salting out agent for effective operation of the process, whereas the product obtained has potentially less desirable properties.
It is an object of certain aspects of the present invention to provide a wet process for producing sodium percarbonate which ameliorates or overcomes one or more of the problems or disadvantages of operating without the benefit of the presence of a substantial concentration of salting-out agent.
It is a second object of further aspects of the present invention to provide a wet process for producing sodium percarbonate having properties rendering it particularly suitable for incorporation in washing compositions.
According to one aspect of the present invention there is provided a continuous process for the manufacture of sodium percarbonate in which hydrogen peroxide, sodium carbonate and an aqueous medium are introduced into a reaction vessel in which the hydrogen peroxide and sodium carbonate react with formation of sodium percarbonate which precipitates out of solution, the precipitated sodium percarbonate is separated from the mother liquor and recovered as product and at least a fraction of the mother liquor is recycled to the reaction vessel characterised in that
in step a) particulate sodium carbonate and recycled mother liquor are fed continuously into a dissolution tank maintained within a temperature range of from 15 to 50C., the relative feed rates and temperature being controlled such that the concentration of sodium carbonate in solution does not exceed 95% of a saturated solution;
in step b) sodium carbonate solution is continuously withdrawn from the dissolution tank;
in step c) the sodium carbonate solution withdrawn from the dissolution tank in step b), additionally or alternatively particulate sodium carbonate and concentrated hydrogen peroxide solution are continuously introduced into an agitated reaction vessel containing an aqueous suspension of sodium percarbonate particles in mother liquor to maintain a mole ratio calculated as H2O2 and Na2CO3 within the range of more than 0.7:1 and less than 1.3:1 maintaining a temperature within the range of from 10 to 25C., thereby continuously forming and precipitating sodium percarbonate.
Such a continuous process for producing sodium percarbonate can be operated without the addition of a salting out agent.
One important feature of the process comprises the restriction of the concentration of sodium carbonate in the dissolution tank to an upper limit of 95% of a saturated solution, and preferably to not more than about 90% of the saturated solution. In many processes, sufficient solid soda ash is introduced into the dissolution tank to increase its concentration to at least 60% of the saturated concentration and in many embodiments desirably at least 70%. The appropriate rate of introduction of sodium carbonate relative to the flow rate of mother liquor can be determined readily, by first determining the concentration of sodium carbonate that constitutes a saturated solution at the selected temperature, measuring its concentration in recycled mother liquor, and by comparison calculating the amount necessary to increase its concentration to within the desired or preferred range. The restriction in the maximum concentration of soda ash introduces as a beneficial consequence a control to prevent undue decomposition of hydrogen peroxide in the mother liquor.
The residence time of the mother liquor in the dissolution tank is controlled so as to attain the desired concentration of soda ash therein. The residence time is often chosen in the range of up to 60 minutes and in some preferred embodiments from 5 to 40 minutes.
A second important feature of the process relates to the control of temperature of the mother liquor in the dissolution tank. The temperature is controlled in association with the feed rates to ensure that the 95% saturation concentration of carbonate is not exceeded. However, it is advantageous that by controlling its temperature to below 35C. and in preferred embodiments to below 30C., it is possible to retard the rate of hydrogen peroxide decomposition significantly by comparison with the conventionally operated dissolution tanks which employ a temperature in the range of 40C. to 55C. In many instances, the dissolution is operated at a temperature of over 15C. and often between 20 and 30C. It will be recognised that the dissolution of soda ash is exothermic, so that it is necessary to cool the mother liquor to prevent the desired temperature being exceeded. Conventional cooling means can be employed such as a cooling jacket or cooling coils. Agitation/mixing of the mixture in the dissolution tank is carried out in practice to enable fresh mixture to contact the cooling surfaces and to reduce the risk of localised excess soda ash concentrations occurring.
By controlling the extent of soda ash introduction to a maximum that is significantly below saturation whilst at the same time maintaining a comparatively low temperature in the dissolution tank, it is possible to achieve a significant increase in the weight of soda ash dissolved in the mother liquor, whilst minimising the risk that fine particulates would remain in suspension or be precipitated from the solution during the dissolution activity, which fines would have the effect of impairing eventual PCS product quality. Consequently, the combination allows the benefit of reducing hydrogen peroxide wastage without creating a significant risk of impairing product quality.
In addition to controlling the temperature of the mother liquor in the dissolution tank, it is beneficial to control it after its separation from precipitated PCS until it is pumped into the dissolution tank so that it is not permitted to exceed 35C. and preferably not exceed 30C., for example by insulating pipework and any intervening holding tank. As a further variation, the mother liquor may be pre-cooled, for example in the transfer pipes or in the holding tank, for example to a temperature of at least 5C. lower than the desired temperature in the dissolution tank, and in some instances between 5 and 20C. By so doing, a lower cooling capacity can be employed in the dissolution tank.
A further important feature of the process comprises the control of the relative rates of hydrogen peroxide and recycled mother liquor into the crystalliser in order to maintain a sub-stoichiometric concentration of hydrogen peroxide relative that in the eventually precipitated PCS, but one that is neither too low nor too high. The use of a mole ratio of below 0.7:1 would increase the risk of in situ precipitation of fine particles of sodium carbonate decahydrate. On the other hand, as the mole ratio of peroxide to soda ash increases, the concentration of PCS in a saturated solution also increases, so that at an excessive mole ratio, an insufficient fraction of the PCS precipitates out recycling a higher concentration of peroxide and increasing the risk of reagent losses during recycling. The selection of a mole ratio of between 0.7 and 1.3:1, and preferably between 0.8 and 1.2:1, particularly around 1:1 balances the objectives of reducing the risk of decahydrate precipitation, maintaining plant capacity and controlling reagent losses on recycling. It will be recognised that the peroxide concentration in the recycled mother liquor produced in a process employing the selected substoichiometric mole ratio of peroxide:soda ash in the crystalliser can be tolerated by virtue of the control of the temperature in the dissolution tank and the controlled extent of soda ash dissolution, so that the various features cooperate together to create a working process. The concentrations of sodium carbonate and hydrogen peroxide in the reaction vessel are desirably monitored, either continuously or at regular intervals.
The crystalliser is usually operated at a temperature selected in the range of from 10 to 25C. and preferably from 15 to 20C. In practice, the operating temperature in the reaction vessel is than the dissolution tank temperature. To achieve the necessary cooling, the vessel is often provided with a cooling jacket and/or cooling coils to obtain and maintain the selected temperature.
The reaction vessel is provided usually with agitation means to maintain the sodium percarbonate particles suspended. Such means can include mechanical agitation or means for pumping the suspension or mother liquor through a submerged pump or via an external loop.
The process can also include a second agitated reaction or buffer vessel interposed between the reaction vessel and the filter/centrifuge. If desired this can be cooled to operate at a lower temperature than in the primary reaction vessel, such between 3 and 8C. lower. The lower temperature can induce further precipitation of sodium percarbonate, thereby enhancing product recovery and reducing the concentration of peroxide in the mother liquor on recycle.
The invention process introduces concentrated hydrogen peroxide as reactant. Desirably, its concentration is at least 30% w/w and is advantageously at least 35 w/w. In practice, its concentration is often not greater than 80% w/w and in many instances not greater than 70% w/w. Excellent results have been obtained employing a concentration in the range of 35 to 60% w/w. The hydrogen peroxide will itself often contain one or more stabilisers for acidic conditions intended to preserve it during transportation or storage prior to its use. Such stabilisers typically include a phosphate such as pyrophosphate, often at a concentration of from 25 to 500 ppm based on the peroxide concentrate, and/or a tin compound which has been introduced as a stannate, but can adopt a colloidal oxy-tin form in situ, often in the range of 10 to 100 ppm based on the peroxide concentrate. The concentrate can optionally contain a polyphosphonate as identified further below, often in a concentration of up to 1000 ppm. The presence of such stabilisers can be taken into account in determining how much additional stabiliser for peroxide or percarbonate is introduced otherwise into the process cycle.
The invention process in this aspect also contemplates the introduction of particulate sodium carbonate into the reaction vessel in some embodiments. The total amount of sodium carbonate introduced in the reaction vessel in solution from the dissolution tank and introduced directly is chosen to provide the mole ratio of sodium carbonate to hydrogen peroxide in the reaction vessel within the ranges described hereinbefore. The choice of the relative proportion of the two modes of introduction remains within the discretion of the process user, ranging from 100% introduction via the dissolution tank to 100% direct introduction. It is beneficial that at least a fraction of the sodium carbonate is introduced via direct introduction, such as at least 10% and in many embodiments from 20 to 60%. By employing direct introduction for at least a fraction, it is possible to compensate wholly or partly for the loss of plant capacity that would otherwise arise from avoiding the use of the common practice salting-out agent. In practice, therefore, the choice of a two mode introduction of soda ash, or substantially all via direct introduction can represent an excellent operating method.
The term direct introduction can be satisfied by feeding the solid material as such into the reaction vessel, or alternatively and preferably by introducing the solids into a stream of liquor fed into the vessel. The liquor can comprise liquor withdrawn from the dissolution tank or possibly liquor withdrawn from the reaction vessel and recycled back to it. In either instance, the point of introduction of the solid material is usually such that little if any of the solid sodium carbonate has dissolved before introduction of the liquor suspension into the tank, but rapid dissolution occurs within the reaction vessel itself.
The soda ash which can be employed can comprise either light or heavy soda ash or a natural material such as trona.
The invention process is often operated at the subsisting alkaline pH achieved by the introduction of sodium carbonate and concentrated hydrogen peroxide to maintain a mole ratio within the ranges specified hereinabove. It remains at the discretion of the operator to vary the pH in the reaction vessel, for example by introduction of soda solution, such as within the range of pH 10 to pH11, or to compensate if the feed should include a fraction of sodium bicarbonate.
It is highly desirable for the mother liquor and/or liquor containing added carbonate withdrawn from the dissolution tank to contain at least one stabiliser for alkaline hydrogen peroxide so as to reduce or minimise its decomposition, and especially in cooperation with the features identified above for controlling decomposition during the production of the PCS product and recycle of the mother liquor. Such stabilisers are often selected from inorganic or complexing stabilisers or from a mixture of both. It will be recognised that stabilisers in the process cycle are usually present in the mother liquor at a higher concentration relative to hydrogen peroxide than in the concentrate.
Alkali and soluble alkaline earth metal silicates represent convenient inorganic stabilisers which co-precipitate in the PCS product and continue to offer stabilisation therein. The silicate is often a sodium, potassium or magnesium silicate or a mixture thereof. The silicate is often represented by the formula Na2O:nSiO2 (or corresponding formulae for other metals) in which n is selected in the range of from 0.5 to 4, such as in ortho or metasilicate. It is convenient to express the amount of silicate employed relative to the weight of soda ash employed, the weight often being selected in the range of from 5 to 80 g/kg soda ash.
Complexing agents for metals, and especially for transition metals represent valuable stabilisers in the invention process. Such complexing agents are often selected from polycarboxylate or polyphosphonate salts, either introduced as such or in acid form, including polyaminocarboxlyates such as EDTA or DTPA, polyaminomethylene-phosphonates such as EDTMPA, CDTMPA and DTPMPA and hydroalkylenephosphonates such as hydroxyethylidenediphosphonate. A convenient amount of such compexing stabilisers to employ is often selected in the range of from 0.5 to 20 g/kg soda ash and particularly from 1 to 5 g/kg.
The point or points of introduction of the stabiliser are at the discretion of the process operator. The stabiliser or stabilisers can be introduced into the crystalliser, either directly or by prior introduction into one or both of the reactant solutions. For example, the silicate can be added together with the soda ash into the dissolution tank and the complexing agent introduced into the peroxide concentrate holding tank. Alternatively, at least a fraction of the complexing agent can also be introduced into the soda ash dissolution tank or into the recycle liquor after its separation from the product and to prior to its infeed into the dissolution tank. The PCS product tends to remove with it a fraction of the complexing stabiliser, so that by restoring its concentration shortly after separation, the loss of peroxide can be further reduced.
It is often advantageous to carry out the production and precipitation of PCS in the presence of a crystal habit modifier, sometimes alternatively referred to as a crystallisation aid. Such modifiers or aids tend to modify the growth of the PCS crystals, encouraging the formation of regular abrasion-resistant needle-shaped crystals and rounded particles. The use of such modifiers can also increase the bulk density of the PCS product, as can complexing stabilisers. The modifiers are often selected from alkali metal (such as sodium) or ammonium phosphates, including hexametaphosphate, pyrophosphate and non-stoichiometric condensed phosphates. Alternative or additional modifiers can comprise homo or co-polymers of acrylate and/or methacrylate, fumarate or maleate and the corresponding acids. The co-polymers include co-polymers of (meth)acrylate/acid with acrylamide and/or alkylene oxides such as ethylene oxide and/or propylene oxide, and can be random or block copolymers.
It is often convenient to employ one or more modifiers to a total weight of from 0.5 to 50g /kg soda ash, and particularly from 1 to 20 g/kg. In some embodiments both a phosphate and a polyacrylate modifier are used, their weight ratio often being selected in the range of from 1:1 to 10:1. in other embodiments, only one type of crystal habit modifier is used, such as the organic polymer, ie the polyacrylate or mixture of polyacrylates. It will also be recoginised that compounds such as organic phosphonates can contribute to crystal habit modification, even if they are nominally introduced as peroxide stabilisers.
In practice, it is highly desirable for the flow rates of materials into and withdrawn from the dissolution tank and the reaction vessel to be balanced, thereby maintaining a substantially steady state, with a constant rate of production of the PCS and substantially constant volumes within the tank and vessel. This can readily be achieved by monitoring the flow of PCS suspension from the vessel and the flows of mother liquor around the cycle and/or the volumes in the tank and vessel and adjusting the pump speeds to control the flow rates accordingly. It can be assisted by including a buffer tank in the mother liquor cycle.
By balancing the respective flow rates of reagents into the vessel and product withdrawn from it, a substantially constant environment can be maintained within the reaction vessel, such as solids density, and concentrations of reagents in the liquor and hence, product characteristics. It is desirable to maintain the concentration of soda ash within the liquor in the reaction vessel at a concentration within the range of 100 to 160 g/kg and preferably within the range of 130 to 150 g/kg. It is similarly desirable to maintain the concentration of hydrogen peroxide in the liquor in the reaction vessel in the range of 25 to 65 g/kg and preferably 35 to 60 g/kg. In practice, the concentrations of soda ash and peroxide in the vessel will also be constrained by the temperature that is maintained in the vessel and the control of the mole ratio of peroxide:carbonate to within the mole ratio range described hereinabove.
At start-up, if mother liquor from a salt-free process is not available, the sodium carbonate can be dissolved to the appropriate concentration in water, which may if desired have been purified or deionised, the hydrogen peroxide introduced into the reaction vessel at a suitable rate to achieve the desired mole ratio (though a higher amount since the residue in recycled liquor is absent) and operation of the process will result in the conditions progressing to the steady state.
The mother liquor introduced into the dissolution tank in step a) can be provided solely by liquor separated from the PCS suspension withdrawn from the reaction vessel.
In a variation of or modification to the foregoing process, and in an additional step h), mother liquor is withdrawn continuously from and returned to the reaction vessel, at least a fraction being recycled via a dissolution tank in step a). The mother liquor is most preferably withdrawn from a non-agitated zone within the crystalliser where particulates can settle, often separated from the agitated zone by a mesh through which mother liquor can pass. The mother liquor recycled in step h) can augment or replace the mother liquor separated from the PCS product in step f). The remaining fraction of mother liquor obtained in steps f) and h) can be recycled directly into the reaction vessel, preferably to the extent that maintains a steady volume. By employing mother liquor extracted directly from the vessel to augment, or even instead of liquor recovered from the suspension, it is possible to recycle the liquor through the dissolution tank at a greater rate than can be provided solely by the liquor recovered from the suspension in step f). This means that it is easier to satisfy the requirement that the concentration of soda ash in the mother liquor in the dissolution tank does not exceed the desired maximum of 95% of the saturation concentration, ie provides a 5% buffer, and likewise is easier to provide the much larger buffer that is preferred. Alternatively or additionally, a fraction of the benefit can be obtained by the increased flow of mother liquor through the dissolution tank enabling a higher plant capacity to be obtained from the same size crystalliser.
Although the process according to the first aspect of the invention and/or the modification above is directed especially to a process operated without addition of a salting out agent, it will be understood that in accordance with a further modification, the process can be operated under otherwise the same operating conditions, but in the presence of sodium sulphate or similar halide-free salting out agent. In such a modification, the mother liquor can contain the halide-free salt in a concentration of often up to 125 g/kg solution, preferably at least 40 g/kg solution and particularly from 60 to 100 g/kg solution.
In accordance with a further modification of the process according to the first aspect, the crystalliser forms an integral part of a crystalliser classifier in which the crystalliser is positioned above and communicates with the classifier, and sodium percarbonate product descends through the classifier to a point of extraction and in counter direction to a stream of liquor passed through the classifier. Such a crystalliser classifier is described more fully in EP-A-0703190, to Solvay Interox SA, which description is incorporated herein by reference. Herein, the crystalliser/classifier is employed in conjunction with the above identified invention conditions for the dissolution tank, namely the controlled introduction of soda ash and its controlled temperature therein so as to minimise or at least reduce the wasteful loss of hydrogen peroxide.
When the crystalliser/classifier is employed in accordance with this modification, it will be recognised that the conditions described in EP-A-0703190 for its operation in the presence of a salting out agent such as sodium chloride are modified to make allowance for the absence of the salting-out agent. Accordingly, it is desirable to maintain the temperature conditions and selection of mole ratio of soda ash to hydrogen peroxide in the crystalliser zone of the crystalliser/classifier within the ranges described hereinabove for a plain crystalliser, but for example using the ascend rate of mother liquor upwardly through the classifier and the relative agitation extent in the crystalliser the classifier described in EP-A-0703190. It will be seen that by employing the combination of the apparatus of EP-A-0703190 in conjunction with the process conditions of the instant invention, it is possible to obtain a product which combines the benefits of both processes, namely a product that is substantially free from salting out agent and has a large tight granulometry, thereby reducing decomposition during storage and incorporation in compositions and also reduces or minimises reagent losses during manufacture.
In this modification using a classifier crystalliser, the benefit is especially attainable by operating a process free from salting out agents, but it will also be recognised that a significant fraction of those benefits can still be retained when a halide-free salting out agent such as sodium sulphate is employed as salting out agent, at the low temperature process operating conditions, the main difference being that the attainable available oxygen in the product is maybe around 0.1-0.2% lower.
According to a second aspect of the present invention, in some embodiments there is provided a process for the manufacture of sodium percarbonate in which hydrogen peroxide, sodium carbonate and an aqueous liquor are introduced into a reaction vessel in which the hydrogen peroxide and sodium carbonate react forming sodium percarbonate which precipitates out of solution, the precipitated sodium percarbonate is separated from the mother liquor and recovered as product and at least a fraction of the mother liquor is recycled to the reaction vessel characterised in that at least a fraction of the sodium carbonate introduced into the reaction vessel is dissolved in mother liquor that has been separated from the precipitated sodium percarbonate and/or otherwise withdrawn from the reaction vessel and the mother liquor containing an enhanced concentration of sodium carbonate is thereafter recycled to the reaction vessel and at least a fraction of the sodium carbonate is introduced in solid form directly into the reaction vessel or into a recycle of liquor withdrawn from and returned into the reaction vessel.
In accordance with the second aspect of the present invention there is provided apparatus for the continuous production of sodium percarbonate which comprises a reaction vessel equipped with an inlet for aqueous hydrogen peroxide and an inlet for sodium carbonate in which vessel the hydrogen peroxide reacts with the sodium carbonate to form sodium percarbonate which precipitates out of solution and provided with means for continuously withdrawing a suspension of sodium percarbonate and passing it to a solid liquid separator, and a line to recycle liquor recovered from the separator to the reaction vessel characterised in that the apparatus includes two means to introduce sodium carbonate into the reaction vessel, one means of which comprises a dissolution tank placed in the line recycling liquor from the separator to the vessel and/or placed in a recycle loop which comprises means for extracting liquor from and returning it to the vessel whereby the sodium carbonate is introduced in dissolved form into the vessel and a second means which introduces particulate sodium carbonate directly into the vessel or into a line introducing liquor into the vessel.
By providing two separate means for introducing the sodium carbonate into the process cycle, greater flexibility is provided for control of the overall process.
In accordance with this second aspect, it will be recognised that such a process in which soda ash is introduced into the process cycle in two different parts is especially well suited to a process which employs either no salting out agent or possibly only a halide-free agent such as sodium sulphate.
In the dissolution tank, the soda ash can desirably be introduced at a rate relative to the influent mother liquor selected to obtain its concentration in the tank and at a temperature in accordance with any of the desired or preferred conditions in step a) of the process according to the first aspect of the present invention. Thus, in particularly desired conditions the sodium carbonate concentration is controlled to between 75 and 90% of a saturated solution and its temperature is controlled to between 25 and 30C. By so doing, this process also retains the benefit of preventing excessive decomposition of hydrogen peroxide during carbonate dissolution. The means for introducing soda ash into the dissolution tank can comprise the conventional means for feeding a particulate material into a tank, such as a conveyor belt, screw, chute or pipework located above the tank and feeding the material from a storage or holding hopper, possibly under the influence of gravity. Naturally, the means includes appropriate flow control means such as valves or gates to control the rate of introduction.
The means for introducing solid soda ash into the process cycle can comprise similar means described above for feeding the particulate material into the dissolution tank. Indeed, it will be recognised that a single flow from the soda ash storage hopper can be split or two separate flows can be taken. However, it can be preferable for the particulate material to be wetted before it is introduced into reaction vessel. This can be achieved by introducing the soda ash into a stream of either mother liquor shortly before it is returned into the reaction vessel. The particulate material can conveniently be introduced via a Venturi device.
The proportion of soda ash introduced in solution and the proportion introduced as a solid can be varied at the discretion of the process user, and indeed may be selected in accordance with the remaining apparatus employed and depending on the other process parameters. If a split soda ash introduction is employed in conjunction with a process operated in accordance with the first aspect, preferably containing no salting out agent, but optionally containing a sulphate, it is desirable in many instances to introduce between 35 to 95%, particularly 45 to 75% via the dissolution tank and the remainder via solid introduction.
The damp sodium percarbonate separated from the mother liquor in accordance with the above-identified processes can be subjected to post-separation treatments including drying, for example in a fluidised bed or rotating bed drier.
Advantageously, the sodium percarbonate produced herein by chloride-free processes, and especially from salt-free processes can and often does exhibit certain particularly desirable properties. In particular, sodium percarbonate is produced which exhibits a very low rate of emission of heat. A representative figure to enable a realistic comparison between products produced using different processes and in different locations can be obtained by first subjecting the percarbonate sample to a 7 day aging process in a sealed ampoule in a constant temperature chamber held at at 40C., thereby bringing the percarbonate to substantially a plateau value for the heat emission. Such aging is indicated herein by reference to the product being 7 day aged. The product is then transferred to microcalorimeter, model LKB 2277, also called a Thermal Activity Monitor which is marketed by Thermometric Limited, Sweden. The heat is measured that is emitted from the sample over a standard period, which herein is 16 hours and at a standard test temperature which herein is 40C. By comparison, a typical product obtained from a wet process involving chloride salting out can often emit from 5 to 7 xcexcW/g in the 16 hour test period, whereas the invention process products usually emit less than 3 xcexcW/g, often at least 0.5 xcexcW/g, and in many instances from 1 to 2 xcexcW/g. By being able to produce a product with such a low heat emission, bull storage and bulk transportation of the product is thereby improved-, lowering and in practice virtually eliminating the likelihood that a self-accelerating decomposition of the product would arise. Sodium percarbonate with a higher heat emission can, of course, be handled and stored safely, but often needs more stringent control and precautionary means to remove the heat evolved. The invention products having lower heat emission can enable the sodium percarbonate to be handled and stored under more adverse conditions, such as in hotter climates or with reduced investment in precautionary means to remove heat.
Additionally, the product of the instant process, normally is produced having a mean particle size of at least 500 xcexcm, often at least 600 xcexcm, and usually not more than 1200 xcexcm and in many instances not more than 1000 xcexcm, and in many preferred instances in the range of from 650 to 850 xcexcm, such as about 750 xcexcm. In other instances, the mean particle size produced falls within the range of 600 to 650 microns. The product usually has a particle distribution which is similar to xe2x80x9cnormalxe2x80x9d, the spread for which depends on the type of plant employed. A crystalliser often produces a product with a spread of around 1 to 1.2 whereas a crystalliser classifier often produces a product with a narrower spread, such as from about 0.6 to about 0.9. The product usually displays at least 80% and frequently at least 90% of its particles by weight within the range of +/xe2x88x9250% of the mean particle size. The advantage of the product having a tight distribution is that it avoids the small particles which are liable to segregation and the worst rate of decomposition and the advantage of a large mean particle size is that this minimises decomposition for example when present in a detergent composition with other current constituents. In some instances, it is desirable to produce products intrinsically comprising mainly particles that are at least 400 microns in diameter and relatively few above 800 microns in diameter.
Desirably, the chloride free invention process described herein can also produce a product which has a low rate of pick-up of moisture from a humid atmosphere. This is demonstrated by a test in which the percarbonate is stored under constant temperature and humidity conditions, such as at 32C. and 80% RH.
In practice the test is conducted using a 9 cm diameter petri dish with a 1 cm depth rim that is weighed accurately on a 4 decimal place balance, (W1). A sample of dry sodium percarbonate (about 5 g) is placed on the petri dish which is gently agitated to generate an even particulate layer across the base of the dish and reweighed on the same balance, (W2). The sample on the petri dish is stored in a room, about 3M high, wide and long in an atmosphere maintained for a period of 24 hours at 32xc2x0 C. by a thermostat controlled heater and at 80% Relative Humidity (RH) by introduction of a fine droplet water spray under the control of an humidity detector and weighed on the same balance, (W3). The samples are protected by a shield from the spray.
The invention products made without chloride salting out agent are observed to pick up less than 30 g/kg in 24 hours in the test, whereas a conventional chloride salted-out product can pick up over 100 g/kg under the same conditions. In many instances, the invention product picks up not more than 15 g/kg in the test, such as less than 15 g/1000 g and in some especially preferred instances below 10 g/kg eg 1-5 g/1000 g. This means in practice that the products of the halide-free invention process are more stable in storage with constituents like siliceous or phosphate builders which it is believed can generate a humid atmosphere.
It is especially preferable in some embodiments in which the crystallised sodium percarbonate is intended to be incorporated in built detergent compositions, such as compositions built with zeolites and/or with phosphates to make the sodium percarbonate by a process in which a classifier is integral with the crystallizer, and operated such that particles above and below a desired minimum size are separated out in the classifier, the larger particles are recovered as product whereas the smaller particles are recycled to the crystalliser where they can grow as a result of deposition of additional sodium percarbonate from solution, typically by addition of salting-out agent into saturated or supersaturated sodium percarbonate solution in the crystalliser, and the enlarged particles flow back into the classifier. Naturally, in accordance with the teaching herein, such salting out agent is free from chloride (except perhaps at an impurity level) in order to provide the advantageous property of low moisture pick-up. Salting out agents, as is known, operate by the common ion effect, so that salting out represents advantageously the addition of additional sodium ions without adding chloride ions to the process solution containing sodium carbonate from which the percarbonate is produced. Such salting out agents can comprise sodium sulphate or like other non-chloride sodium salt in an integrated crystalliser/classifier and particularly one in which mother liquor is caused to flow upwardly through the classifier into the attached crystalliser that is positioned above. Such combined classifier/crystallisers are particularly beneficial in that by suitable operational control, it can be possible to control the granulometry of the particles during manufacture rather than having to employ an external and hence additional classification process whilst at the same time producing the sodium percarbonate with advantageous properties such as low moisture pick-up and low heat emission described herein.
Advantageously, by the use of the processes according to the present invention, and particularly those variations which omit a halide salting out agent from the process cycle, it is possible to produce products which have a high purity, for example having an Avox of at least 14.5% and in the absence of any salting out agent, a product in some embodiments which has an avox of at least 14.8%.
The presence of selected peroxide stabilisers and crystal habit modifiers in the mother liquor during the precipitation means that in general they are also co-precipitated in the product. Their presence can affect not only the habit of the crystals but also the size distribution, and it is believed that they contribute significantly to the excellent properties of the resultant product.
It has also been found that the invention process products retain the excellent rates of dissolution and high bulk density that have previously been exhibited by products that were obtained in a wet process using chloride to salt out.
According to a fourth aspect of the present invention, there is provided dry particulate sodium percarbonate characterised in that intrinsically
a) it has a mean particle size of at least 500 xcexcm up to 1200 xcexcm and
b) it has a 7 day aged heat emission in 16 hours of below 3 xcexcW/g.
xe2x80x9cIntrinsicallyxe2x80x9d herein refers to the sodium percarbonate itself, even if it is subsequently subjected to one or more treatments such as surface coating or agglomeration.
Such a product enjoys the twin benefits of large particle size enabling it more readily to be employed for its most common use, viz incorporation in built washing compositions or built additive compositions and especially those containing siliceous and/or phosphate builders, and of low heat emission enabling it to be transported and stored in bulk at the point of manufacture or use, such as under more adverse conditions or with less heat control investment.
The fourth aspect in preferred embodiments provides sodium percarbonate which can be further characterised by one or more of the following features:
c) It is produced by crystallisation from a bulk solution containing hydrogen peroxide and sodium carbonate;
d) It is produced in a process free from chloride or preferably any salting-out agent;
e) It has a moisture pick-up over 24 hours storage at 80% relative humidity and at 32C. of not more than 30 g/kg and preferably not more than 15 g/kg;
f) It has an apparent bulk density of from 800 to 1100 g/kg, and preferably from 850 to 1000 g/kg;
g) It contains at least one phosphonate stabiliser and at least one crystal habit modifier selected from sodium silicate and a polyacrylate in a total amount of from 1 to 8 g/kg;
h) It has an Avox (available oxygen) of at least 14.5% and preferably at least 14.8%.
By producing the product from a bulk solution, it is possible to employ plant that has previously been used for making other persalts, thereby introducing flexibility into manufacture and prolonging the life of plant.
The advantage of avoiding chloride salting out agent is that it avoids the effect of chloride on the rate at which moisture can be picked-up and increases the chance of attaining a high avox.
By producing a product having intrinsically a low rate of moisture pick-up, the stability is maintained of the product in humid conditions, such as appertain in detergent and especially zeolite-built compositions.
By producing a product having a bulk density that is like that conventionally produced in a wet process involving chloride salting out, the product can be readily substituted for existing PCS.
By selecting a product which contains the selected stabiliser and/or crystal habit modifier in the range shown, the product demonstrates improved crystal habit and stability compared with when such products are absent.
By producing a product which can have a high avox, there is greater activity provided per unit weight.
In a number of embodiments, desirable percarbonate products comprise products which are obtained using a process that excludes a salting-out agent and are further defined by one or more, preferably two or more and especially three or more of features selected from a), b), and e) to h) described hereinabove.
The properties of products produced in the invention process and specified in the fourth aspect of the invention can be further enhanced by further treatments. Such treatments can include granulation and surface treatments with one or more inorganic or organic coating agents to respectively bind the PCS particles together or and/or interpose a layer between the sodium percarbonate and its environment. Such a granulation process often involves contacting the PCS particles with an aqueous solution of a binder under low agitation conditions which encourage the particles to remain bound together. Such coating processes are often conducted under more abrasive conditions that discourage agglomeration, and usually involve the PCS absorbing an aqueous or non-aqueous solution or slurry of the coating agent, followed by drying in similar apparatus to the above dryers, or by contact with a substance that melts or softens, adheres to the PCS surface and is thereafter cooled. A very large number of suitable coating agents is known already. The inorganic agents include treatment with one ore more agents selected from alkali metal carbonate and/or sulphates, boric acid and/or alkali metal borates, alkali metal phosphates, alkali metal silicates, or polysilisic acids. A particularly desirable coating is based on the use of mother liquor, preferably containing additional coating agent, such as selected from the list of inorganic coating agents given above, or after prior concentration. Use of mother liquor in this manner in many instances reduces the amount of surplus mother liquor that would otherwise need to be discharged to waste or otherwise treated.
A coating with or containing an alkali metal chloride can also be contemplated, especially where the PCS is likely to be stored or employed in a dry atmosphere. It will be recognised that at least a fraction of the alkali metal salts can often be replaced by a magnesium salt, at the discretion of the user. Amongst organic coating agents can be contemplated polycarboxylates, hydroxycarboxylates, polyalkyleneaminocarboxylates and polyalkylenephosphonates, many of which have been classified as chelating agents, employable as chelating builders in detergent compositions. Examples include sodium citrate, sodium tartrate, sodium gluconate, EDTA, DTPA, and ethylenediamine tetramethylene phosphonate. Mixtures of the various inorganic and organic coating agents can be employed.
The PCS produced by a process according to the present invention, either as such or when subjected to subsequent treatments such as coating, can be employed for any of the uses hitherto proposed or adopted for particulate PCS. The products are particularly suitable for incorporation in particulate washing compositions, such as those summarised hereinbelow, for example in an amount up to about 30% w/w and often from 2 to 20% w/w, w/w herein indicating by weight based on the composition.
The washing composition can contain the other constituents that have been proposed or adopted. Such washing compositions are often targeted towards fabric washing, dishwashing or general purpose hard-surface cleansing. The other main constituents in such compositions comprise surfactants, often chosen in an amount of from 3 to 40% w/w and in many instances from 5 to 25% w/w, builder, often chosen in an amount of from 1 to 60% w/w and in many instances from 5 to 40% w/w, and adjuvants which often total no more than 20% w/w. The adjuvant often includes one or more germicides, soil anti redeposition agents, optical brighteners, antifoaming agents, colorants and perfumes, in up to a small amount for each, which is often less than 2% w/w. A further important adjuvant comprises a bleach activator, which can be present in an amount of usually not more than about 5% w/w, often 1 to 3% w/w and are in many instances O-acyl or N-acyl compounds which react with PCS to generate a peracid or similar peroxygen compound in alkaline solution or transition metal complexes, often of manganese, iron or cobalt. Some compositions can include a fabric softener, often in an amount of up to about 10% w/w, which is often a cationic surfactant deployed on a clay support. A further constituent of standard compositions, a bulking agent or processing aid, usually sodium sulphate or sodium chloride, can also be present, often in an amount of 0 up to about 70% w/w.
Suitable surfactants include soaps and synthetic surfactants which are often either anionic such as alkyl benzene sulphonates, olefin sulphonates, linear alkyl sulphonates, alcohol sulphates and other sulphated materials such as sulphated glycerides, ethers, sulphosuccinates or phosphate esters, and fluoralkylsulphonates or nonionic such as alcohol ethoxylates, alkylphenol ethoxylates polyethylene oxide/polypropylene block copolymers, and condensates of fatty acids or amides with aliphatic polyols such as sorbitol. The weight ratio of anionic to nonionic surfactants is often in the range of 5:1 to 1:2. Other classes of surfactant which can be present, but usually to a lesser extent, include amphoteric, zwitterionic and cationic surfactants. Suitable cationics are often quaternary ammonium, phosphonium or sulphonium compounds.
Builders which can be employed in the washing composition are often chosen from two categories, inorganic builders and complexing organic builders. Inorganic builders include alkali metal condensed phosphates, and particularly tetrapyrophosphate, tripolyphosphate and metaphosphates, alkali metal borates, alkali metal carbonates and siliceous builders including alkali metal silicates, layered silicates such as products available under the trade designation SKS6, clays such as bentonite and especially zeolites such as zeolites A, X and Y and MAP zeolites. The PCS produced by the invention processes herein that are free from halide-salting out agent are particularly suitable for employment in conjunction with the most aggressive builders towards PCS, namely the zeolites.
The complexing organic builders often are selected from alkali metal polycarboxylates or polyaminocarboxylates or polyalkylenephosphonates. Examples include citrate, carboxylated starch derivatives; nitrilotrisodium triacetate, EDTA; EDTMP and DTPMP.
The alkali metal builder is frequently a sodium salt.
Where the sodium percarbonate and builder and/or diluent and/or bleach activator are formulated into dry bleach compositions, the sodium percarbonate often constitutes from 10 to 90% w/w and the other constituents the remainder. The builder and diluent are each often selected in the range of from 10 to 80% w/w and the activator is often selected from 0 to 10% w/w. The builder and/or diluent and/or activator can be selected from the lists of materials described hereinbefore for washing compositions.
Having described the invention in general terms, specific embodiments thereof are described in greater detail by way of example only.

Sodium percarbonate and process for producing sodium percarbonate


The present invention relates to a hierarchical system and method for task assignment (TA), coordination and communication of multiple Unmanned Aerial Vehicles (DAV's) engaging multiple attack targets and conceives an ad-hoc routing algorithm for synchronization of target lists utilizing a distributed computing topology. Assuming limited communication bandwidth and range, coordination of UAV motion is achieved by implementing a simple behavioral flocking algorithm utilizing a tree topology for target list routing. The TA algorithm is based on a graph-theoretic approach, in which a node locates all the detectable targets, identifies them and computes its distance to each target. The node then produces an attack plan that minimizes the sum of distances of the UAV's in the subtree of a given node to the targets.

The problem of design, development and control of multi-agent systems has been studied in recent years for many applications. In particular, the use of systems consisting of multiple autonomous robots or unmanned aerial vehicles (UAV's) has been proposed in order to meet the requirements of complex missions. Control, communication and decision support systems for UAV's constitute rapidly evolving research and development fields, as indicated by the Department of Defense UAV Roadmap 2002-2027. The use of groups of cooperating UAV's in order to perform various missions is currently studied throughout the world and is considered a main research goal by the United States Air Force Research Laboratory (AFRL).
Using a cooperative group of UAV's for autonomously attaining a given goal requires that each agent assume a certain task at a given time. The global result of the group of UAV's acting together is the execution of a mission, e.g. search and attack, comprising the tasks of search (look for a suspected target), identification (determine that the suspected target is a legitimate target), track (update the target location) and attack (launch munitions).
Assigning multiple UAV's to perform these tasks according to their capabilities is a challenge that requires the development of specialized algorithms. These algorithms may be classified into two main types: optimal and heuristic. While optimal algorithms yield better results in terms of task assignment, they are usually more sensitive to system uncertainties, enemy behavior, and environment changes. Heuristic algorithms, on the other hand, are usually sub-optimal but more robust. It would be desirable to develop a heuristic task assignment (TA) algorithm, which is robust to changes in the UAV group properties. An issue strongly related to cooperative UAV motion is flocking (also referred to as formation flying). Prior art simulated biological flocking behavior is based on cohesion (agents converge onto a given point), alignment (velocity matching in order to move at a give direction), collision avoidance (preventing an agent from colliding with other agents), obstacle avoidance (steering the agents away from obstacles) and migration (path following). It would be highly desirable to implement a behavioral flocking model for UAV's in order to avoid collisions and improve mission execution by facilitating wide theater coverage.

Carmel-Haifa University Economic Corporation Ltd.

Technion Research and Development Foundation Ltd.


A bone plate includes an upper surface, a bone contacting surface, and at least one hole extending through the upper and bone contacting surfaces for receiving a bone screw. The at least one hole includes a first portion and a second portion that overlap one another. The first portion defines a substantially circular outer periphery and the second portion defines an elongated outer periphery that is elongated in a direction substantially parallel to the longitudinal axis of the plate. The first portion may have threads configured to engage threads on the head of a bone screw, and the second portion may be configured to cooperate with the head of a different bone screw to provide compression to the fracture.

The present invention relates generally to devices for fixation of parts of a fractured bone and more specifically, to bone plates and systems for stabilization and/or compression of parts of a fractured bone.
Bone plates may generally be utilized to carry out two different types of osteosynthesis, namely xe2x80x9crigid osteosynthesisxe2x80x9d and xe2x80x9cflexible osteosynthesis.xe2x80x9d Rigid osteosynthesis is used for medical care of joint fractures, simple shaft fractures (where nailing is impossible) as well as for osteotomies. Aside from the possibility of anatomical repositioning, the bone itself supports and stabilizes the osteosynthesis, which allows for the possibility of putting stress on the extremity earlier and without pain. Additional advantages of the medical care of stable fractures can be observed when the blood circulation in the bone is greatly diminished due to trauma. For treating xe2x80x9cnonunionsxe2x80x9d or in the case of existing infection, the fracture must be kept stable in order to make bone healing possible and so as not to irritate the infection further by instability of the fracture gap.
Flexible osteosynthesis, also known as xe2x80x9cbiological osteosynthesis,xe2x80x9d may be desirable in the medical treatment of comminuted fractures in the shaft region of tubular bones. In the case of these fractures, it is an objective to maintain the proper length of the bone and to fix the bone ends (joints) in their proper anatomic positions with respect to one another. With flexible osteosynthesis, the fracture zone is not directly affixed or manipulated, and consequently, the blood circulation in this area is not inhibited. Bone plates designed for flexible osteosynthesis thus operate similarly to a locking, intramedullary nail, which is anchored only in the metaphyses.
Since fractures cannot always be treated with one type of osteosynthesis, surgeons must frequently compromise because a bone plate, which allows him to combine the two types of osteosynthesis discussed above, is not available. Such a combination would be beneficial, for example, when a joint fracture can be compressed with traction screws through the bone plate and the whole of the joint may be connected to the diaphysis over an internal fixative with angularly stable screws. Another illustrative application concerns porotic bones, where a bone plate with axially and angularly stable screws can be anchored in the metaphysial fragment, with a stable plate-affixation being undertaken in the diaphyseal range with the assistance of a plate traction screw through the fracture. A primary fracture stabilization can be achieved by this type of procedure.
This situation has led to the development and marketing of bone implants for both types of osteosynthesis. The two types of implants, however, are designed specifically for their respective method. Thus, the disadvantages of these two systems lie in the difficulty in combining them.
Thus, a need exists for improved bone plates that provide for both rigid and flexible osteosynthesis.
The present invention is directed to a bone plate that is adapted to be used for both rigid and flexible osteosynthesis, without compromising the ability of the plate to be used for either type of osteosynthesis. Accordingly, the bone plate of the present invention may be used as a compression plate or as an internal fixative.
This objective is accomplished with a bone plate having at least one xe2x80x9ccombination hole.xe2x80x9d The combination hole may be used with a screw having a substantially spherical head to provide for compression of the fracture, or may be used with a screw having a threaded head to fix the position of the screw with respect to the bone plate and serve as an internal fixative.
The combination hole includes a first portion and a second portion that at least partially overlap one another. The first portion may be substantially circular, and the second portion may be elongated. Within the scope of the invention, the second portion (elongated portion) may have a diametrical dimension that is greater in one direction than in another. For example, the diameter of the elongated portion may be greater in the direction of the longitudinal axis of the plate than in the direction substantially perpendicular to the longitudinal axis. Thus, the elongated portion may be oval, elliptical, rectangular or any other elongated shape known to one of ordinary skill in the art, including combinations of these shapes. The diameter (D) of the first portion (circular portion) may be smaller than the minor (or shortened) axis (B) of the second portion (elongated portion). Typically, diameter (D) is between about 5% and about 25% smaller than the minor axis (B).
According to another aspect of the invention, the circular portion of the hole may be configured and dimensioned to engage the head of a bone screw. More specifically, the circular portion may be provided with an internal thread or a peripheral lamella or lip that may engage a corresponding structure formed on the screw-head. In the case where an internal thread is provided, the thread may be disposed in a single plane, or in several planes. The plane(s) may be parallel to the upper and/or bone contacting surfaces of the bone plate. According to one embodiment, the internal thread may extend over the whole height of the bone plate from the bone contacting surface to the upper surface. This configuration provides increased stability of the bone plate/screw-head interface.
With the threaded screw-head engaged in the threads of the first portion, the bone plate may be used as an internal fixative. Use in this configuration, however, creates high stresses at the interface of the bone plate and screw-head because the plate is not forced against the bone, and therefore, the bone fracture is fixed primarily by friction between the plate and the bone. This increase in stress is taken into account by the threaded portion of the hole extending over a range of at least about 180xc2x0 with respect to a central axis of the hole, and thereby enclosing the screw-head in at least this angular range. This feature of the bone plate is especially advantageous where thin bone plates are involved. Preferably, the threaded portion is disposed on one of the two longitudinal ends of the hole. This positioning allows for the threaded portion to extend over a larger angular range. For example, the threaded portion may extend over a range of between about 190xc2x0 and about 280xc2x0, and preferably over a range of between about 200xc2x0 to 250xc2x0, thus maximizing the strength of the bone screw to bone plate interface.
According to another embodiment of the invention, the internal thread may be tapered (i.e., formed on the inner surface of a hole that tapers with respect to its central axis). Preferably, the internal thread tapers radially inward toward the bone contacting surface of the bone plate. A bone screw to be rigidly fixed to the bone plate may include a screw-head having a tapered external thread (i.e., formed on an outer surface of the screw-head that tapers with respect to the central axis of the screw-head) that is tapered to match the shape of the tapered internal thread. The bone screw may be rigidly fixed to the bone plate by engagement between the matching threads. This method of attachment is especially advantageous when self-drilling screws are to be used since, due to the tapered shape of the matching threads, the screw may be inserted into the bone independently of the plate. More specifically, the screw-head becomes rigidly clamped to the plate only as the threaded screw-head penetrates the threaded portion of the hole. Despite any initial misalignment between the threads on the screw-head (the position of which are initially dictated by the orientation of the bone screw in the bone) and the threads on the bone plate, the tapered shape of the mating threads ensures that the threads on the screw-head will ultimately align with the threaded portion of the hole. When the tapered thread of the screw-head is tightened into the internal thread of the hole, the screw-head creates radial forces in the plate hole. Thus, the bone plate must be dimensioned and configured to withstand these high radial forces, e.g., to withstand flexing of the walls of the screw holes in the bone plate.
According to one embodiment, the inner thread conically tapers at a cone angle of between about 5xc2x0 and about 20xc2x0. Preferably, the thread tapers at a cone angle of about 10xc2x0.
In the case where the inner thread is tapered, as discussed above, the thread may extend through a different angle when measured at the upper surface than when measured at the bone contacting surface. For example, when measured at the upper surface, the inner thread may extend through a first angle of between about 180xc2x0 and about 230xc2x0, while when measured at the bone contacting surface, the inner thread may extend through a second angle of between about 230xc2x0 and about 270xc2x0.
The first portion of the hole, and consequently the inner thread (if provided), may be oriented closer to the center or intermediate portion (as distinguished from the ends) of the plate than the second portion, thus avoiding any undesirable effects on the compression capability of the second portion. Thus, when the bone plate is used as a compression plate, the geometry of the second portion (compression portion) is not adversely affected by the presence of the internal thread.
According to another aspect of the present invention, at least one of the holes may be dimensioned and configured to receive a substantially spherical head of a bone screw and provide for compression of the fractured bone fragments. For example, according to one embodiment, the second portion of the elongated hole, discussed above, may include a concave, substantially spherical recess at the upper surface. The recess may be dimensioned and configured to accommodate the spherical head of a conventional bone screw. Such an arrangement may be especially useful when the bone screw is put in place eccentrically with respect to the hole, as is necessary for attaining compression of a fracture. Additionally, the second portion of the hole may flare outward in the area of the bone contacting surface to provide for increased angulation of the bone screw with respect to the bone plate.
According to another embodiment of the invention, the underside of the bone plate may be concave, thus allowing the plate to conform to the rounded cross-section of the tibia, femur, humerus, forearm bone, and other bones with which the present invention may be used. The concave configuration of the underside also allows a conventional bone screw to be inserted obliquely through the plate hole. This feature may be especially important when gripping a small bone fragment, which must be pulled against the plate.
The present invention is also directed to a bone plating system including at least one bone screw. The bone screw may have a screw-head that is configured and dimensioned to engage the circular portion of the above-described combination hole. For example, the screw-head may include a plurality of external threads disposed thereon that engage the internal threads of the bone plate.


A process for producing a composite article from at least one polar thermoplastic, in particular an engineering thermoplastic, and from at least one foamed or unfoamed polyurethane elastomer, where a molding is firstly molded from the thermoplastic, and some or all of this is provided with an adhesion promoter based on acrylic-resin lacquer or polyurethane-resin lacquer comprising solvents, and then a coating which may be in full-surface or strip form or at least one other molding made from the polyurethane elastomer is used for overmolding, and the thermoplastic material is thus bonded to the polyurethane material.

The invention relates to a process for producing a composite article from polar thermoplastics and polyurethane elastomers, in which the thermo-plastic material is treated with an adhesion promoter comprising solvents, and also to the composite articles which can be produced and are produced thereby. Use of the adhesion promoter makes it possible to obtain adhesion of polyurethane elastomers on plastics which do not exhibit any adhesion without use of the adhesion promoter.
The polar thermoplastics include in particular those known as engineering thermoplastics, also known as high-performance polymers. These have excellent thermal, mechanical and chemical properties and are therefore preferred materials for engineering applications. Moldings produced from these have good strength and hardness, together with excellent resilience, and are therefore applied widely and very frequently in all areas of daily life, for example in the construction of motor vehicles and other equipment, and for housings and casings, keyboards and snap connectors. Their excellent sliding-friction properties also make them useful for many moving parts, such as power train components, deflector rolls, gear wheels and shift levers.
However, moldings of this type frequently have a low mechanical damping factor, which in certain applications requires the use of soft damping elements. In addition, it is often necessary to use a seal at junctions when moldings are installed. The high surface hardness of the moldings can, under certain circumstances combined with a low sliding-friction coefficient, cause items placed thereon to slip, and this can compromise, for example, the operating reliability of switching units and control units. For these reasons, combinations of hard and soft materials are used more and more frequently in order to combine the particular properties of these materials with one another. The hard material here is intended to give the component or molding its strength, and the elastic properties of the soft material permit it to take on the functions of sealing, or of damping vibration or deadening sound, or of altering surface feel. In these applications it is important that there is adequate adhesion between the hard and the soft component.
Gaskets, damping elements, etc. are frequently provided separately, made from a soft material, and usually anchored mechanically to a thermoplastic molding or adhesive-bonded thereto in an additional operation, creating more work and in some cases considerable cost.
A newer and more cost-effective method is multicomponent injection molding, in which, for example, a second component is used for overmolding on a first, ready-molded, component. The adhesion between the two components here should be very strong, but may frequently be further improved in physical interlocks, by applying undercuts, for example. Good basic adhesion between the two components, for example as a result of chemical affinity, is generally a precondition for their practical application and is therefore of great importance for this process.
Examples which are well known are multicomponent-injection-molded combinations of polypropylene with polyolefin elastomers or with styrene-olefin elastomers, or of polybutylene terephthalate with polyester elastomers or with styrene-olefin elastomers. Polyamides, too, adhere to very many soft components.
There are also known moldings made from polyacetal with directly molded-on functional elements, produced using uncrosslinked rubbers (DE-C 44 39 766). The bond strength of composite articles of this type is, however, still unsatisfactory.
DE-A9611272 describes just such composite articles, composed, inter alia, of a polyacetal, a rubber copolymer, a reinforcing filler, a crosslinking agent and, if desired, other conventional additives. Particularly good adhesion of the polymer components is achieved here by vulcanizing the rubber fraction. However, the elevated temperatures and long times needed for vulcanization make this additional step disadvantageous.
Another application (German patent application No. 197 43 134.8) relates to a process for producing composite articles from polyacetal and a soft component, by preinjecting the polyacetal in a mold in a first step and, in a second step, using the lower-hardness material for overmolding, thereby bonding this to the polyacetal. For the lower-hardness region here, use is made of a thermoplastic polyurethane elastomer (TPE-U) with a hardness of from Shore A 65 to Shore D 75. However, this range of hardness is too high for many applications. In addition, the thermoplastic polyurethane elastomers described have the known disadvantages in processing, e.g. moisture absorption, the resultant thermal instability and variable flowability and also mold-release problems.
It is clear that softer damping elements could be obtained using foamed elastomers, which generally have a lower hardness than the corresponding compact elastomers. In processes which have become established in recent years, therefore, soft elastomer foam gaskets are applied, in a second operation, to the hard material which has been removed from an injection mold. Single-component or two-component polyurethanes are particularly suitable for this purpose. Addition-crosslinking two-component polyurethanes are particularly advantageous for achieving high production rates, as a result of relatively rapid and thorough curing without the need for prolonged drying times. However, the different chemical nature of these substances and the different processing conditions for the hard and soft component frequently give rise to adhesion problems in such cases.
The method generally used for improving adhesion to thermoplastics is to flame-treat the surface of the plastic, or to treat it with some other oxidative process, e.g. corona treatment or plasma treatment.
It is also well known that an adhesion promoter can be used to bring about adhesion between materials which do not per se adhere to one another. The makeup of the adhesion promoter here must be such that it has high chemical affinity for both of the materials to be bonded to one another, but does not subject these materials to chemical attack.
Whereas solutions have been found for a wide variety of cases, for example the adhesion of a plastic to metal, glass or other materials, there is a difficulty with particular engineering thermoplastics in achieving sufficiently strong and durable adhesion to polyurethane elastomers. However, it is precisely these engineering thermoplastics which are of interest for many applications, for example in automotive construction, where a molding, component or functional part produced from these thermoplastics is to be provided with a gasket. Polyurethane elastomers are again particularly suitable for gaskets of this type.
The object was therefore to provide a process which avoids the disadvantages and limitations mentioned and produces a composite article made from engineering thermoplastics and polyurethane elastomers.
Surprisingly, it has now been found that the use, as an adhesion promoter, of clear lacquers or primers which have been designed for use on metal surfaces gives an adhesive bond between polyurethane elastomers and engineering thermoplastics or other polar thermoplastics. Suitable adhesion-promoter systems involve acrylic-resin lacquers and polyurethane-resin lacquers which comprise solvents and which may have been modified by adding fillers, pigments or a crosslinking component.
By using these adhesion-promoter systems according to the invention, sealing or damping elements, for example, made from polyurethane elastomers can be laid down with adhesive bonding directly onto moldings made from polar thermoplastics, or be molded onto these, without any need for undercuts or other assembly steps. Without using these adhesion-promoter systems it is not possible to produce a bond between the polyurethane elastomer and, in particular, polyacetal or a polyester. Even widely used techniques, such as surface-etching, plasma treatment (corona discharge) or flame-treatment of the surface of the thermoplastic molding, do not give the desired result.
The invention therefore provides a process for producing a composite article from at least one polar thermoplastic and at least one foamed or unfoamed polyurethane elastomer, where a molding is firstly molded from the polar thermoplastic, and some or all of this is provided with an adhesion-promoter system based on acrylic-resin lacquer or polyurethane-resin lacquer comprising solvents, and then a coating which may be in full-surface or strip form or at least one other molding made from the polyurethane elastomer is used for overmolding, and the thermoplastic material is thus bonded to the polyurethane material via the adhesion promoter.
The invention also provides, in particular, the composite articles which can be produced by the novel process.
The novel composite article here is formed by a thermoplastic molding, some or all of which has been coated with a polyurethane elastomer, or onto which one or more moldings, also called functional parts, made from a polyurethane elastomer have been directly molded on. This may be, for example, a sheet-like molding, one side of which carries a layer made from polyurethane elastomer, or a molding of any desired shape on which the polyurethane elastomer has been laid down in the form of strips, sometimes called beads.
Examples of novel composite articles are generally functional parts or moldings provided with gaskets, damping elements, retaining elements and other functional elements etc. The functional parts and moldings obtain the required dimensional stability from the thermoplastic material and the desired frictional properties, sealing function, acoustic properties, feel and appearance from the elastomer coating. They include, for example, housings and closure systems, and also bodywork parts and motor vehicle parts in the widest sense.
The composite article may, however, also be composed of one or more thermoplastic moldings of any desired shape, onto which have been directly molded one or moldings of any desired shape made from the polyurethane elastomer. The expression xe2x80x9cdirectly molded onxe2x80x9d means, for the purposes of the present invention, that the functional elements have been directly overmolded onto the molding with which they are intended to enter into a strong adhesive bond, in particular by multicomponent injection molding.
In every case, the adhesion between the thermoplastic material and the polyurethane elastomer is brought about by the adhesion-promoter system according to the invention, which is applied to the thermoplastic molding before the polyurethane elastomer is used for overmolding.
The adhesion-promoter system according to the invention is generally an acrylic-resin lacquer polyurethane-resin lacquer comprising solvents. If desired, the lacquer may have been modified using fillers, such as talc, heavy spar or zinc oxide, or also using pigments, in particular functional pigments, enabling, for example, low coefficients of thermal expansion and good adhesion to polar and nonpolar surfaces to be established. The solids content in the lacquer is usually more than 40% by weight, frequently even more than 50 or 60% by weight, and is matched to the desired conditions for bonding and processing.
The solvents present in the adhesion-promoter system are essentially organic, i.e. aliphatic and/or aromatic solvents, and it may be advantageous here to use mixtures of different solvents. Examples of suitable solvents are butyl acetate, xylene, ethylbenzene and benzyl alcohol, and also mixtures of these. The solvent firstly gives the lacquer its workability, for example for painting or spraying, and also slightly softens the contact surface to allow deep penetration of the functional constituents of the adhesion-promoter system into the microstructure of the hard thermoplastic substrate.
According to the invention it is preferable to use a lacquer with urethane-crosslinking acrylic resin. This lacquer may be used on its own or together with a crosslinking component which crosslinks hydroxyl or amino functions present in the acrylic resin. The crosslinking component added may, for example, be a bi-, tri- or multifunctional isocyanate. The amount of crosslinking agent to be used here depends on the amount and functionality of the acrylic-resin components to be crosslinked, and also on the desired degree of crosslinking.
The particular feature of the adhesion-promoter system to be used according to the invention is that it can generally be used on polar and nonpolar surfaces, i.e. in particular on many thermoplastics having polar groups. The adhesion-promoter system can be matched to particular hard/soft components (hard=thermoplastic, soft=elastomer) by varying the nature and amount of fillers, pigments, solvents and crosslinking agents. The adhesion-promoter system adheres well to polyurethane, since chemical similarity and polar groups predominate. A further advantage is that after drying, i.e. after evaporation of the solvent, a matt surface remains on the adhesion promoter applied, and this surface additionally brings about mechanical anchoring of the polyurethane elastomer.
The composite article is produced by well known methods and processes. A cost-effective and advantageous process is one in which the thermoplastic molding is first produced, for example by injection molding, followed by coating with the adhesion promoter, and the polyurethane elastomer is then laid down as a bead or in full-surface form, or molded on in the form of a molding. It is possible here to lay down the polyurethane elastomer on the surface treated with the adhesion promoter before this has completely finished reacting and/or has dried. The adhesion promoter may advantageously be applied by spraying, as is usual in the paint industry. The soft polyurethane material used is advantageously in the form of two components and may, in particular if it is to be laid down in the form of a bead, advantageously be applied using mixing and metering machinery for low output rates (from about 0.1 to 100 g/sec).
The tensile bond strength achieved by the novel process between the thermoplastic molding and the polyurethane elastomer generally depends on whether the polyurethane elastomer used is in foamed or unfoamed form. For unfoamed polyurethane elastomer, a desirable bond strength for many applications is at least 0.3 N/mm2, and for foamed polyurethane elastomer a bond strength of from 0.1 to 1.0 N/mm2 is achievable. Higher tensile stresses are likely to lead to tearing of the elastomer. For functional partsxe2x80x94depending on the loadingxe2x80x94relatively high adhesion is desired.
For the purposes of the present invention, adhesion between the polar thermoplastic, in particular the engineering thermoplastic, and the polyurethane elastomer is present if it is predominantly cohesive fracture which occurs when these two constituents of a composite article are pulled apart, i.e. if during this the constituents of one component remain adhesive-bonded to the other. If the polyurethane elastomer can be pulled away from the thermoplastic without residues of the elastomer remaining on the thermoplastic, or, conversely, residues of the thermoplastic remaining on the elastomer, the fracture is adhesive and the adhesion is inadequate.
The polar thermoplastics according to the invention generally include polymers with polar functional groups in the main chain, e.g. in the structural building blocks of the chain, or in substituents, but include in particular those known as engineering thermoplastics. Among the engineering thermoplastics, for many applications particular preference is given to high-performance polymers which have a melting point above 100xc2x0 C., in particular above 200xc2x0 C. High-performance polymers are described, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, 5th edition, VCH Verlagsgesellschaft mbH, Weinheim-New York 1992, and in G. W. Becker, D. Braun: Kunststoffhandbuch Vol. 3/3, Carl Hanser Verlag, Munich 1994, which are incorporated herein by way of reference.
According to the invention, the engineering thermoplastics include particularly polyacetals, polyesters, polyarylene sulfides, polycarbonates, polyphthalamides, polyetherimides, polyether ether ketones, syndiotactic polystyrene (SPS), cycloolefin copolymers and liquid-crystalline polymers (LCPs). The thermoplastics may be used individually or as a mixture, or also as a blend with one or more other plastics which are thermoplastic, thermosetting or elastomeric. In particular, the thermoplastics may also comprise other polymeric additives, for example in order to modify their chemical or mechanical properties for a particular application, or in order to improve their adhesion properties.
As well as freshly produced products, the engineering plastics produced may also be recycled materials of the 1st, 2nd or higher generations, or also mixtures of freshly produced products and recycled materials, which may, if desired, comprise additives and/or have been modified by admixing other compatible polymers.
Polyacetals are well known and are described, for example, in DE-A 29 47 490.They are generally what are known as polyoxymethylenes (POMs), which generally contain at least 80 mol%, preferably at least 90 mol%, of oxymethylene units (xe2x80x94CH2Oxe2x80x94). For the present invention, polyoxymethylenes are either homopolymers of formaldehyde or of its cyclic oligomers, such as trioxane or tetroxane, or else appropriate copolymers made from formaldehyde or from its cyclic oligomers, in particular trioxane, and from cyclic ethers, cyclic acetals and/or linear polyacetals.
The polyacetals used generally have a melt index (MFR 190/2.16) of from 0.5 to 75 g/10 min (ISO 1133). It is also possible to use modified grades of POM, such as blends made from POM with TPE-U (thermoplastic polyurethane elastomer) or with MBS (methyl methacrylate-butadiene-styrene core-shell elastomer), or with methyl methacrylate-acrylate core-shell elastomer, or with PC (polycarbonate), or with SAN (styrene-acrylonitrile copolymer) or with ASA (acrylate-styrene-acrylonitrile copolymer composition).
Polyesters preferably contain polymerized units which derive from an ester of at least one aromatic dicarboxylic acid, in particular terephthalic acid, isophthalic acid or 2,6-naphthalenedicarboxylic acid, and of at least one linear aliphatic dialcohol, in particular ethylene glycol, 1,3-propanediol or 1,4-butanediol, or contain polymerized units which derive from tetrahydrofuran. Polyesters of this type are described, for example, in xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d, ed. Barbara Elvers, Vol. A21, chapter on polyesters (pp. 227-251), VCH, Weinheim-Basel-Cambridge-N.Y., 1992.The polyesters may be either homopolym or else copolymers of these building blocks. Particular preference is given to polybutylene terephthalate and polyethylene terephthalate, and also to poly(butylene terephthalate-co-butylene isophthalate).
The polyester raw materials may also have been modified to incorporate by condensation small proportions of aliphatic dicarboxylic acids, e.g. glutaric acid, adipic acid or sebacic acid, or of polyglycols, such as diethylene glycol, triethylene glycol or else higher-molecular-weight polyethylene glycols, or may also contain polymerized units which derive from hydroxycarboxylic acids, preferably hydroxybenzoic acid or hydroxy-naphthalenecarboxylic acid.
Polyarylene sulfides are also known, inter alia, as polyarylene thioethers. These include heat-resistant polymers built up from arylene sulfide groups. The arylene units here are based on mono- or polynuclear aromatic compounds. Examples of these are phenylene, biphenylene, naphthalene, anthracene and phenanthrene, with single or multiple substitution if desired. Preferred polyarylene sulfides are polyphenylene sulfides, known, for example, by the trade names (copyright)Fortron and (copyright)Ryton.
Polycarbonates are described, for example, in xe2x80x9cEncyclopedia of Polymer Science and Engineeringxe2x80x9d, John Wiley and Sons, New York 1988, Vol. 11, pp. 648-718, which is incorporated herein by way of reference. Polycarbonates are supplied, for example, with the name (copyright)Makrolon (Bayer AG).
Cycloolefin copolymers are described by H. Cherdron, M. Brekner and F. Osan in Die Angewandte Makromolekulare Chemie (223), 121, 1994, which is incorporated herein by way of reference. Cycloolefin copolymers are supplied, for example, with the name (copyright)Topas (Ticona GmbH).
Polyether ether ketones (PEEK) are available commercially from Victrex Deutschland GmbH, liquid-crystalline polyesters are, for example, available with the name (copyright)Vectra (Ticona GmbH), and polyetherimides are available with the name (copyright)Ultem (General Electric).
According to the invention it is possible to use either single-component or else two-component polyurethane elastomers. It is particularly advantageous to use an addition-crosslinking, two-component flexible polyurethane foam system. The polyurethane elastomer used according to the invention is composed of polyhydric short-, medium- or long-chained alcohols (polyols) built up on a polyester basis or on a propylene oxide-ethylene oxide copolymer (ether polyol) basis, with hydroxyl end groups crosslinked using multifunctional isocyanates.
The A component of a two-component flexible foam system comprises, besides the polyols mentioned, water, catalysts and other additives conventional in polyurethane processing. It may also comprise conventional stabilizers, processing aids or additional coupling agents, for example on the basis of modified silanes, or also from 0 to 70% by weight of inorganic fillers.
The B component of a system of this type is composed predominantly of bifunctional and higher-functional isocyanates. These are usually aromatic isocyanates, such as diphenylmethane diisocyanate (MDI) or tolylene diisocyanate (TDI), or aliphatic isocyanates. By varying the makeup, and also the mixing ratio, of the A and B components it is possible to produce modified, foamed or unfoamed, polyurethane elastomers with a variety of properties. The term polyurethane elastomers here does not include what are known as thermoplastic polyurethane elastomers (TPE-Us).
Either the thermoplastic material or the polyurethane elastomer may generally also comprise conventional additives. Examples which should be mentioned here are stabilizers, nucleating agents, mold-release agents, lubricants, fillers, reinforcing materials, pigments, carbon black, light stabilizers, flame retardants, antistats, plasticizers and optical brighteners. The additives may be present in amounts which are general and usual.
Besides the application sectors mentioned at the outset, the novel composite articles are used as connecting elements in the form of fittings, couplings, rollers, bearings, functional parts with integrated sealing and/or damping properties, and also as elements which are non-slip and easy-grip. These include, in particular, housings used in automotive construction, such as door closure housings, window lifter housings, sliding-roof sealing elements and the like, and also central-locking units, internal and external trim, mudguards, bumpers, etc., and also fuse boxes and electrical distribution systems, power train housings, engine housings, housings for separating wet and dry spaces, and moreover fastening elements with an integrated seal, such as clips with sealing rings or sealing washers, decorative strips with an integrated sealing lip, sealing elements for compensation in expansion joints, fastening elements with good damping properties, e.g. clips with centers for damping vibration or noise, power train components, such as gear wheels with damping elements, gearboxes with integrated flexible couplings, non-slip, easy-grip elements, such has control levers or control knobs, or grip surfaces on electrical devices, on tools or on writing implements, etc., and also chain links with a resilient surface.

Composite article from engineering thermoplastics and polyurethane elastomers, using an adhesion promoter