T 0046/05 07-06-2006

I. Mention of the grant of European Patent No. 0 678 535 in the name of Asahi Kasei Kogyo Kabushiki Kaisha, later Asahi Kasei Chemicals Corporation, in respect of European patent application No. 95 201 699.6, a divisional application of European patent application No. 91 902 766.4, filed on 17 January 1991 and claiming priority of a Japanese patent application No. JP 26 641 490 dated 5 October 1990, was announced on 23 June 1999 (Bulletin 1999/25) on the basis of 4 claims.

Claim 1 read as follows:

"1. A process for producing a polyoxymethylene copolymer which comprises copolymerizing a mixture of a cyclic oligomer of formaldehyde or a cyclic acetal as the principal monomer and a comonomer selected from cyclic ethers and cyclic formals, in the presence of a perfluoroalkylsulfonic acid or a perfluoroalkylsulfonic acid derivative as the polymerization catalyst in an amount of 5x10-6to 2x10-5% by mole relative to the principal monomer to obtain a copolymer which contains at least 0.07% by mole of polyoxyalkylene units in proportion to polyoxymethylene units, wherein the content of water and formic acid in the monomer mixture are, respectively, not more than 40 ppm by weight".

Claim 2 restricted the content of comonomer in the polymerization mixture to 0.3 to 3.5 % by mole of the principal monomer.

Claim 3 read as follows:

"3. A process according to any one of claim 1 or 2, in which the comonomer is selected from cyclic ethers represented by the formula

FORMULA/TABLE/GRAPHIC

and cyclic formals represented by the formula

FORMULA/TABLE/GRAPHIC

in which formulae m is 2 to 6 and R and R0 may be the same or different and R groups bound to different carbon atoms and R0 groups bound to different carbon atoms may be the same or different from each other, and are selected from hydrogen, alkyl groups and aryl groups."

Claim 4 specified a further step of deactivating the catalyst.

II. A notice of opposition against the grant of the patent was filed on 22 March 2000 by Ticona GmbH, invoking the grounds pursuant to Article 100(a) EPC, in particular that the subject matter claimed in the patent lacked novelty (Article 54 EPC) and was not founded on an inventive step (Article 56 EPC).

Eight documents were cited in the notice of opposition, inter alia:

E1: PL 90 091 (cited in the form of an English translation);

E2: US 3 580 928

E3: US 4 312 977

E4: US 4 431 794.

A further document, designated E9 was submitted by the proprietor in its response to the notice of opposition, dated 31 October 2000.

(nb during the proceedings before the opposition division, the prefixes "E" or "D" were variously, and inconsistently employed to designate the documents, the relevant index numbers however remained constant. In this decision the prefix "E" will be employed).

In a subsequent submission of 23 March 2001, after said response of the proprietor to the notice of opposition, six further documents, designated E10-E15 were cited, inter alia:

E10: K. Weissermel et al "Polymerization of Trioxane", Angewandte Chemie, International Edition, 1967 (6), No. 6, pp 526-533;

E11: US 3 607 882;

E12: US 4 332 644.

In its last submission prior to the oral proceedings before the opposition division (a letter dated 19 July 2004) the opponent invoked for the first time the grounds of opposition pursuant to Article 100(c) EPC. Further two additional documents, E16 and E17 were cited.

In a decision announced orally on 21 September 2004 and issued in writing on 4 November 2004 the opposition division rejected the opposition.

With regard to the documents E10-E15 and E16 and E17 the decision stated:

"The Patentee objected to the Opponent's filing of additional documents in general outside the nine month opposition period and to the admission of D16 and D17 in particular. In the event these documents were not discussed in detail and were not formally admitted to the procedure".

Although the ground pursuant to Article 100(c) EPC had been invoked subsequent to filing of the notice of opposition, the Opposition Division considered it to be potentially sufficiently serious to warrant admission pursuant to Article 114(1) EPC. With regard to this ground, the decision held that the wording "selected from cyclic ethers and cyclic formals" did not constitute a broadening of the subject matter of the application since the application as originally filed taught, albeit in a discussion of the background art, that these were the comonomers conventionally employed in the art.

Regarding novelty, it was held that none of the documents cited disclosed all the features of the claims. In particular E1 was silent on the level of formic acid. The statement in E1 that the trioxane was recrystallised and freshly distilled could not be interpreted as disclosing that the trioxane would exhibit the necessary level of formic acid, since from E2 it was unclear that such purification would automatically lead to a decrease in the formic acid content. The argument of the opponent that the inventor of E1 would have used monomer purification techniques known from E2 or E3 and that these purification techniques would inherently lead to a reduction in formic acid level were dismissed.

Regarding inventive step, it was held that E1 represented the closest prior art, the subject matter claimed being distinguished therefrom by the specified maximum content of formic acid in the monomers. The technical problem was formulated as being to provide polymers with improved heat stability, heat aging resistance and hot water resistance. The use of pure monomers and the impact of impurities such as water and formic acid on the copolymers, in particular the activity as chain transfer agents, was known from a number of documents. The data in the patent was held to show that both the initiator employed and the impurity content of the monomers influenced the properties of the copolymers obtained. E1 taught that the process was tolerant of impurities other than water. However E1 contained no teaching to employ monomer feeds of low formic acid content in order to obtain polymers with lower formate end group contents and improved distribution of comonomer units leading to polymers having improved thermal stability.

Accordingly the opposition was rejected.

III. An appeal against this decision was filed by the opponent on 7 January 2005, the appropriate fee being paid on the same day.

IV. The statement of grounds of appeal were filed on 11 March 2005. The objections in respect of extension beyond the content of the application as filed (Art 123(2) EPC) and lack of inventive step (Article 56 EPC) were maintained.

(a) With regard to the documents E10-E15 and E16 and E17, it was submitted that the position taken in the decision (cited above) was not clear and appeared only to exclude E16 and E17.

With regard to E10 to E15 it was submitted that these documents had been filed as a reaction to the proprietor's response to the opposition. The proprietor had had ample opportunity to take position on these and had in fact done so. There was no formal request to deem these documents inadmissible.

(b) With regard to Article 123(2) EPC it was argued that the wording in claim 1 "…selected from cyclic ethers and cyclic formals…." was derived from a discussion of the background art and should not be read as part of the disclosure of the invention. This wording constituted a generalisation intermediate between the most general and most specific disclosures of the invention in the application as filed.

(c) Concerning inventive step, it was held that E1 was the closest prior art, the claimed subject matter being distinguished therefrom by the specified content of formic acid. The subjective problem was, as formulated in paragraph [0008] of the patent the production of polyoxymethylene copolymers having both good stability, and good mechanical properties. Previously known catalysts could have a degradation effect on the polymer, adversely affecting the molecular weight and stability. E1 taught to overcome these deficiencies by using the catalyst defined in the claims of the patent. However the claimed subject matter was associated with no unexpected technical advantage, hence the objective problem was to provide an alternative process.

It was generally known in the art that formic acid had a deleterious effect on the polymerization and the art taught to purify trioxane to remove or at least reduce the amount of formic acid. Further it was a common desideratum to produce polymers with as high a molecular weight as possible. It was known from E10 that water and carboxylic acids will reduce the molecular weight of the polymer. Hence the skilled person would seek to keep as low as possible the concentration of materials capable of acting as chain transfer agents, such as water or formic acid, and so employ highly purified monomers. The statement in E1 that the process was tolerant of impurities did not preclude the use of highly pure monomers.

It was further submitted, with reference to documents disclosing methods for purifying trioxane (E2, E3, E11, E12), that it would be a matter of obviousness to keep the starting monomers as free of impurities as possible.

It was disputed that there was any link between the features of impurity level and type and amount of catalyst and that these merely represented a collocation of known features that would have been routinely adopted by the skilled person.

It was also disputed that the claimed catalyst or the features of the subsidiary claims could support an inventive step.

V. In its response dated 21 September 2005 the respondent (proprietor) submitted first and second auxiliary requests for the case that the objection pursuant to Article 123(2) EPC should be held to be well founded.

Claim 1 of the first auxiliary request corresponded to a combination of claims 1 and 3 as granted, in that the comonomer was defined as specified in granted claim 3. Claim 3 of this request corresponded to granted claim 4. Claim 1 of the second auxiliary request restricted the comonomer to certain specifically named compounds, stated to be derived from pages 5 and 6 of the application as filed. In this request also claim 4 as granted became claim 3.

(a) It was requested that the documents filed outside the opposition period not be formally admitted to the procedure. If these documents were to be admitted, remittal to the opposition division was requested.

(b) It was requested to reverse the decision admitting the ground of opposition pursuant to Article 100(c)/123(2) EPC.

Regarding the substance of this objection it was submitted that the fact that the wording in question was present in a discussion of the background art did not detract from the fact that it formed part of the information content of the application as filed and would be read by the skilled person as an indication of the field of the invention.

(c) Regarding inventive step the position of the appellant was disputed. In particular it was argued that since the process of E1 was disclosed as being tolerant of impurities, there was no incentive to seek extremely pure materials.

It was further submitted that the combination of the catalyst and the impurity level were linked. The low level of catalyst reduced the effect of hydride shift while the vulnerability of the catalyst to loss of activity had been minimised by controlling the impurity level of the trioxane. Thus these two features worked together to provide a process with unexpected advantages.

VI. The Board issued on 10 February 2006 a summons to attend oral proceedings. In the accompanying communication, it was provisionally held that the ground of opposition pursuant to Article 100(c) EPC had been admitted by the opposition division and hence formed part of the appeal proceedings. It was further provisionally held that the objections pursuant to Article 123(2) EPC in respect of the main request were well founded. No conclusion could be reached in respect of the first auxiliary request due to certain discrepancies between the wording thereof and that of the claims as originally filed. Auxiliary request 2 was provisionally held to meet the requirements of Article 123(2) EPC.

Regarding novelty it was provisionally held that the amount of catalyst disclosed in examples II and III of E1, when related to the principal monomer as required by claim 1 of the patent as granted, fell outside (above) the claimed range. Thus the claimed subject matter was provisionally held to be distinguished from the disclosure of E1 by the content of formic acid and by the catalyst concentration.

VII. In a letter dated 30 March 2006 the respondent filed new main and first auxiliary requests which, it was stated, had been amended to address the objections pursuant to Article 123(2) EPC.

Claim 1 of the main request read as follows:

"1. A process for producing a polyoxymethylene copolymer which comprises copolymerizing a mixture of a cyclic oligomer of formaldehyde or a cyclic acetal as the principal monomer with a cyclic ether or cyclic formal as comonomer, in the presence of a perfluoroalkylsulfonic acid or a perfluoroalkylsulfonic acid derivative as the polymerization catalyst characterised in that the perfluoroalkylsulfonic acid or perfluoroalkylsulfonic acid derivative is present in an amount of 5x10-6 to 2x10-5% by mole relative to the principal monomer to obtain a copolymer which contains at least 0.07% by mole of polyoxyalkylene units in proportion to polyoxymethylene units, and the content of water and formic acid in the monomer mixture are, respectively, not more than 40ppm by weight."

Claim 1 of the first auxiliary request differed from claim 1 of the main request in that the phrase "with a cyclic ether or cyclic formal as comonomer" was replaced by the wording:

FORMULA/TABLE/GRAPHIC

"and a comonomer which is a cyclic ether represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6; or

"and a comonomer which is a cyclic ether represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6; or

FORMULA/TABLE/GRAPHIC

a cyclic formal represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6;" .

a cyclic formal represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6;" .

The subsidiary claims remained as previously noted.

It was submitted that the objections pursuant to Article 123(2) EPC had been overcome by recasting claim 1 of the main request into the two part form.

The amendments to the auxiliary request were directed to eliminating the discrepancies highlighted by the board in the communication.

VIII. With a letter dated 3 May 2006 the appellant argued with respect to the late filed documents that the statement in the decision "these documents" referred only to E16 and E17 but that E10-E15 had been admitted into the procedure. As emerged from paragraph 2.6.5 of the decision E10, E11 and E12 had been taken into account by the opposition division.

The appellant further maintained the objections pursuant to Article 123(2) EPC with respect to the main request. It was disputed that the reformulation of the claim overcame this objection.

A further, new, objection pursuant to Article 123(2) EPC was raised in respect of all the requests regarding the feature that the content of polyoxyalkylene units in proportion to polyoxymethylene units be at least 0.07 % by mole.

It was submitted that the only disclosure in the application as filed was of the range of 0.07 to 0.5% by mole of inserted oxyalkylene units relative to 100 moles of oxymethylene units. There was no basis for an open ended definition of this feature. Further page 3, line 54 of the A-document stated that copolymers containing more than 0.5% of oxyalkylene units had "markedly poor" mechanical properties.

Regarding inventive step, in the light of the comments by the board relating to E1 it was submitted that E1 no longer constituted the closest prior art since it differed both in respect of the amount of catalyst and the formic acid content. It was furthermore considered not to be a good springboard to the invention because of the teaching - deemed to be "isolated and incorrect" - that "generally products of a lower degree of purity… may be used".

The appellant submitted that instead E4 was the closest prior art document. The sole example of this document disclosed all the features of claim 1 with the exception that it did not explicitly (emphasis of the appellant) state that the concentrations of water and formic acid were less than 40ppm. The problems to be solved by the patent were formulated as being insufficient molecular weight and inadequate thermal stability due to the formation of unstable terminal structures. These problems were considered to be connected since higher molecular weight polymers would have proportionally fewer terminals and hence higher thermal stability. E2 taught to solve this problem by reducing the impurity content to about 40ppm.

IX. In a letter dated 10 May 2006 the respondent objected to the introduction of a further objection pursuant to Article 123(2) EPC, and requested that if this be admitted to the procedure, the respondent be accorded the right to formulate further auxiliary requests, either in advance of, or at, the oral proceedings.

X. Oral proceedings were held on 7 June 2006.

(a) The respondent confirmed that the second auxiliary request as filed in the response to the statement of grounds of appeal was maintained.

In the course of the oral proceedings a further four requests were filed (third to sixth auxiliary requests).

Claim 1 of the third auxiliary request was modified compared to claim 1 of the main request by restricting the proportion of polyoxyalkylene units in proportion to polyoxymethylene units to the range of 0.07% to 1.02% by mole. The wording of claims 2-4 corresponded to that of the equally numbered claims of the main request.

Claim 1 of the fourth auxiliary request, restricted this range to 0.07% to 0.5% by mole. The wording of claims 2-4 of this request corresponded to that of the equally numbered claims of the main request.

The fifth auxiliary request combined the features of the first and fourth auxiliary requests. Claim 1 thereof thus read as follows:

FORMULA/TABLE/GRAPHIC

"1. A process for producing a polyoxymethylene copolymer which comprises copolymerizing a mixture of a cyclic oligomer of formaldehyde or a cyclic acetal as the principal monomer and a comonomer which is a cyclic ether represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6; or

"1. A process for producing a polyoxymethylene copolymer which comprises copolymerizing a mixture of a cyclic oligomer of formaldehyde or a cyclic acetal as the principal monomer and a comonomer which is a cyclic ether represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6; or

FORMULA/TABLE/GRAPHIC

a cyclic formal represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6;

a cyclic formal represented by the formula wherein R and R0 may be the same or different and are each selected from hydrogen, an alkyl group and an aryl group, R bound to different carbon atoms and R0 bound to different carbon atoms may respectively be the same or different and are selected from hydrogen, an alkyl group and an aryl group, and m is 2 to 6;

in the presence of a perfluoroalkylsulfonic acid or a perfluoroalkylsulfonic acid derivative as the polymerization catalyst in an amount of 5x10-6 to 2x10-5 % by mole relative to the principal monomer to obtain a copolymer which contains 0.07% - 0.5% by mole of polyoxyalkylene units in proportion to polyoxymethylene units, and wherein the content of water and formic acid in the monomer mixture are, respectively, not more than 40 ppm by weight."

The wording of claim 2 of this request corresponded to that of granted claim 2 while the wording of claim 3 corresponded to that of granted claim 4.

Claim 1 of the sixth auxiliary request differed from claim 1 of the fifth auxiliary request by restricting the principal monomer to trioxane. The subclaims 2 and 3 were as noted for the fifth auxiliary request.

(b) Regarding the late filed documents E10-E15, the appellant submitted these should be admitted and considered. These had been filed in response to the answer of the proprietor to the opposition and were relevant, being directed to the arguments relating to inventive step, in particular to demonstrate common general knowledge. The filing of these did not represent an abuse. The decision showed that E10, E11 and E12 had been considered, which indicated that all of E10-E15 had been admitted.

The respondent submitted that the fact that the documents had been considered did not mean that a decision was taken to admit these. Further the fact that the respondent had presented arguments to take account of these documents was irrelevant to the question of whether they should be admitted. It was submitted that the opposition division did not rely on these documents in its reasoning, despite the mention of certain of these in the decision.

After deliberation, the board informed the parties that E10 to E15 were in the procedure.

(c) Regarding the admissibility of the ground pursuant to Article 100(c) EPC,

(i) the respondent submitted that this had not been invoked during the nine month opposition period. As the opposition division had decided on the substantive aspects of this issue in the proprietor's (respondent's) favour, it had had no opportunity to appeal against the admission of this ground. The Board was still in a position to rule this objection inadmissible due to having been filed late. The second objection, raised in the letter dated 3 May 2006 was in any case too late, and having not been admitted was not yet part of the procedure. It was submitted that if the ground was introduced then the case should be remitted to the opposition division.

(ii) The appellant submitted that the most recently filed objection pursuant to Article 123(2) EPC constituted a new argument but not a new ground. This change in the case could be admitted at the board's discretion (Article 10b(1) RPBA). The objection was not complex and could be dealt with swiftly.

(iii) After deliberation the board informed the parties that the ground of Article 123(2) EPC was already in the proceedings and that the new argument under this ground was admitted.

(d) Regarding the substance of the objection pursuant to Article 123(2) EPC

(i) The appellant submitted that the terms "polyoxymethylene" and "polyoxyalkylene" were synonymous with "oxymethylene" and "oxyalkylene" respectively. There was no express basis for the open ended definition of at least 0.07% by mole. Page 3 line 49 of the A publication disclosed a closed range.

(ii) The respondent submitted that the cited passage gave support for an open ended range. The lower limit was associated with heat resistance - below this threshold heat resistance was poor. The upper value represented the upper limit at which good mechanical properties were obtained. Both properties were optimised within the range. The patent was based on a divisional application for which, in contrast to the parent application, mechanical properties were irrelevant. Only heat resistance was important. This could be seen from page 20 of the documents for grant ("Druckexemplar") from which the phrase "and tensile strength was excellent" had been deleted. The irrelevance of the upper limit of 0.5 for the divisional application was further supported by examples 32 and 33 of the granted patent which had been comparative examples in the application as originally filed and the parent application. With regard to a statement in paragraph [0008] of the patent that the amount of inserted oxyalkylene units be restricted to a certain range, the respondent submitted that this was prefaced by "The present inventors have found", which meant that the statement should not be interpreted as relating restrictively to the subject matter of the invention.

(iii) Following deliberation the board informed the parties of the conclusion that the main and first and second auxiliary requests did not meet the requirements of Article 123(2) EPC. The respondent submitted the above mentioned third and fourth auxiliary requests. The appellant argued that hereby a substantial limitation of the claims had been made which had never been considered either by the examination or opposition division and could possibly be relied upon by the respondent in respect of its submissions on inventive step. Hence it was submitted, with reference to Article 10b(3) RPBA that these amendments should not be admitted. The respondent submitted that the amended requests should be admitted, being a reaction to a late filed objection by the appellant. It further submitted that the amendment was made solely in order to address the objection raised pursuant to Article 123(2) EPC, and it was not intended to rely on this feature in the discussion of inventive step.

Following further deliberation the board informed the parties that the third and fourth auxiliary requests were admitted to the procedure.

(iv) Regarding the substantive issues in respect of the third auxiliary request, the appellant objected to the upper limit of 1.02% by mole of oxyalkylene units, arguing that the only basis for this was a comparative example in the application as originally filed. It was further submitted that it was inadmissible to extract this feature from the comparative example in isolation of the other features thereof.

The respondent submitted that although the example in question had been indicated to be a comparative example in the divisional application as originally filed, the text (description and examples) of the divisional application being identical to that of the parent application, the invention to which the divisional application related was different from that of the parent (see section X.d.ii above). As a consequence, the labelling of certain examples had changed during prosecution. The example in question had been a comparative one as regards the mechanical properties, but not in respect of the heat resistance properties. There was no reason to assume that the compositions of examples 32 and 33 were not valid for the invention of the divisional application as a whole. In any case it was submitted that the labelling of an example did not affect the disclosure thereof.

(v) Regarding the fourth auxiliary request the appellant raised no objections to the range of the content of oxyalkylene units. However it was submitted, as set out in the written proceedings, that the definition of the comonomer contravened Article 123(2) EPC since the only basis for this was in a discussion of the prior art.

The respondent submitted that the opposition division had accepted this wording. With regard to the decision G 1/93 (OJ EPO 1994, 541) it was argued that this amendment merely limited the protection conferred but did not constitute a change in the technical teaching. It was also submitted that this amendment gave the respondent (proprietor) no unwarranted advantage. This did not constitute an intermediate generalisation but employed the wording of the application as filed, albeit in a discussion of the background art.

(vi) Following announcement by the board that the third and fourth auxiliary requests did not meet the requirements of Art. 123(2) EPC the respondent submitted the above mentioned fifth and sixth auxiliary requests.

(vii) Regarding the fifth auxiliary request the appellant raised no objections pursuant to Article 123(2) EPC.

(e) Objections pursuant to novelty (Article 54 EPC) were not raised.

(f) Regarding inventive step the appellant considered E4 to represent the closest prior art. The subject matter of claim 1 of the fifth auxiliary request differed from the example of E4 due to the specified content of water and formic acid in the monomers. The properties of the polymer obtained - Melt Index (190ºC, 2.16 Kg) of 4 g/minute and a thermal stability in nitrogen (2 hours, 220ºC) of 99.8% demonstrated very good thermal stability but low molecular weight. The technical problem according to the patent was to improve heat stability and mechanical properties. It was obvious to solve this by increasing the molecular weight, which increase could be achieved in an obvious manner by reducing the content of formic acid and water in the monomers. E2 taught a process for purifying trioxane to yield impurity levels of less than 40 ppm, which monomers yielded high molecular weight polymers due to the reduction in chain transfer reactions. A similar teaching was derivable from E11 and E12. The review article E10 showed why reducing water and formic acid led to improved thermal stability since proton donors acted as chain transfer agents, leading to a reduction in molecular weight.

Regarding the teaching in E1 that the process was tolerant of impurities it was submitted this was an isolated teaching and could not play any role in the light of the teachings of E4, E2 and E11.

The respondent submitted that the conclusions reached by the opposition division remained valid even though the identity of the closest prior art had changed. E1 and E4 were more or less equivalent. There was no teaching in E4 to reduce the water and formic acid content. The patent claimed a specific catalyst in a specific low amount in combination with a low amount of defined impurities. The key teaching in E1 was that the process was tolerant of impurities other than water. There was no teaching relating to formic acid. The objective problem was, as before, to improve heat and hot water stability. E4 provided no teaching to reduce the amount of water but taught to employ water as a regulator in an amount of 100-3000 ppm.

Regarding the teachings relating to purification of trioxane, it was not disclosed that such highly pure trioxanes would be useful with the catalysts defined in the claim. The improvements promised by E2 related to different catalysts and were not needed by the process of E1 or E4. Although E1 taught distillation and recrystallisation as adequate purification techniques the teaching of E2, in the context of a different catalyst system, was that these process would not yield sufficiently pure monomer. Hence E2 taught a purification which was not necessary when employing the catalyst of E1, E4 and the patent.

XI. The appellant (opponent) requested that the decision under appeal be set aside and that the European patent No. 678 535 be revoked.

The respondent (proprietor) requested that the decision under appeal be set aside and that the patent be maintained on the basis of the main request or, in the alternative, of one of the first to sixth auxiliary requests in that order.