METHOD FOR THE PRODUCTION OF A POLYAMIDE MOULDING

Abstract: The invention relates to a method for the production of a polyamide moulding that contains a filler, wherein a composition which contains at least one monomer suitable for forming a polyamide is allowed to polymerise in a mould in the presence of the filler, characterised in that the composition is fed to the mould under elevated pressure and in that the mould is brought under reduced pressure before the composition is fed in. The method according to the invention results in the production of completely compacted mouldings which are pore-free and gas-tight and have a uniformly distributed matrix. One of the advantages of the invention is that relatively large mouldings can be produced which, inter alia, can be used as a structural element.

Description: Method for the production of a polyamide moulding The invention relates to a method for the production of a polyamide moulding that contains a filler, wherein a composition which contains at least one monomer suitable for forming a polyamide is allowed to polymerise in the presence of the filler in a mould. A method of this type is disclosed in US 3 418 268. This patent describes a method for the preparation of a polyamide moulding that contains 5 to 95 % (m/m) of an inorganic filler. The polyamide moulding is prepared by adding liquid lactam and adhesion improver to filler. Activator and catalyst are then added to this mixture and the mixture is mixed well. The mixture obtained is heated to above the initiation temperature in order to initiate the polymerisation and the mixture is then rapidly poured into a mould where the mixture polymerises rapidly. A disadvantage of this method is that the viscosity of the mixture of filler, lactam, activator and catalyst which has not yet polymerised becomes higher the higher the filler content. The high viscosity restricts the speed at which the mould can be filled. Very high filler contents are consequently not feasible. Moreover, as a result of this it is possible only to fill moulds of relatively small size. Another method which describes the production of polyamide mouldings is disclosed in NL 1 003 609. This publication describes a method for the preparation of a polyamide moulding with which a component which consists of lactam, catalyst and activator and a component which consists of filler are poured simultaneously or virtually simultaneously, independently of one another, into a mould. This method prevents the mixture that is poured into a mould from becoming too viscous. However, the method according to NL 1 003 609 has the disadvantage that the lactam-containing component and the filler-containing component are poorly mixed. This disadvantage is further exacerbated because, of the two components, the filler component must be at a temperature which is above an initiation temperature needed to initiate the polymerisation. Moreover, the mould must be at a temperature below the initiation temperature, which means that the mould has to be cooled between two operations, which is not economical. The component which contains lactam, catalyst and activator must be at a temperature which is lower than the initiation temperature. If there is inadequate mixing this leads to inhomogeneous polymerisation and therefore to mouldings which have adverse characteristics. With this method as well the disadvantages become more serious as the mouldings increase in size. Moreover, the method according to NL 1 003 609 has the disadvantage that air and moisture present in the mould can become included. This also can lead to mouldings which have adverse and non-constant characteristics, air inclusions and/or increased porosity. The aim of the invention is to provide a method with which mouldings can be prepared which have a high filler content without the abovementioned disadvantages arising with this method. Another aim of the invention is to provide a method with which relatively large mouldings can be produced. These aims are achieved according to the invention by means of a method for the production of a polyamide moulding that contains a filler, wherein a composition which contains at least one monomer suitable for forming a polyamide is allowed to polymerise in a mould in the presence of the filler, characterised in that the composition is fed to the mould under elevated pressure and in that the mould is brought under reduced pressure before the composition is fed in. By means of the method according to the invention a homogeneous mixture is rapidly obtained in which completely homogeneous polymerisation can take place. In addition, bringing the mould under reduced pressure prevents moisture and air from being able to influence the polymerisation process or from being able to be included in the product. Initially the mould is kept under pressure during the polymerisation, as a result of which topping up is effected. The method according to the invention results in the production of completely compacted mouldings which are pore-free and gas-tight, with a uniformly distributed matrix. The difference in pressure in the method according to the invention, where a mould is filled under elevated pressure whilst the mould is brought under reduced pressure, has significant advantages for the production of polyamide mouldings compared with known methods of filling, where a mixture is poured into the mould or where a mould is filled by means of a mixing head. The mouldings obtained are found to have substantially improved material properties compared with the mouldings obtained according to the prior art, such as described, inter alia, in NL 1 003 609 and US 3 418 268. In addition it is possible, using the method according to the invention, to produce larger mouldings with the same favourable characteristics than has been possible to date. With the method according to the invention a more rapid and more flexible production process is achieved and the method according to the invention also gives access to a wider range of matrix and filler materials with preselected properties. As described above, the composition which is fed in under elevated pressure contains at least one monomer suitable for forming a polyamide. Preferably this monomer is a lactam. Lactams which have at least five atoms in the ring, such as, for example, α - pyrrolidone, ε -caprolactam, C-substituted caprolactam, capryllactam, laurinolactam or mixtures of the cited lactams are suitable for the method according to the invention. Since amongst the polyamides nylon 6 has the highest modulus, ε -caprolactam is preferably used. In combination with the lactam, an activator and catalyst are preferably used in order to effect the polymerisation. The method according to the invention can be carried out using a known moulded nylon catalyst. Known moulded nylon catalysts are sodium hydride, sodium alkanolate, alkali metal lactamate and alkaline earth metal bis-lactamate. Examples of these are sodium caprolactamate, potassium caprolactamate, magnesium bis-caprolactamate and caprolactam-magnesium halide. Sodium caprolactamate is preferably used. The use of chlorine-containing catalysts, activators or flame extinguishers is not to be recommended when producing structural elements which contain steel reinforcement. The method according to the invention can be carried out using an activator which is known for the polymerisation of lactams. The customary concentrations of 0.1 - 2 % (mol) can be used. Compounds which supply suitable activator groups are: (poly)isocyanates, polyacyl-lactams and lactam-terminal polyisocyanates. The (poly)isocyanates can be aliphatic, araliphatic, cycloaliphatic and aromatic isocyanates. Examples of suitable isocyanates are hexamethylene diisocyanate (HDI), xylylene diisocyanate (XDI), isophorone diisocyanate (IPDI), tolylene diisocyanate (TDI), methylenebis(phenyl isocyanate) (MDI) and hydrogenated TDI, XDI or MDI, modified MDI (for example with carbodiimide). Examples of polyisocyanates are biurets and trimers of aliphatic diisocyanates, such as 1,4-butane diisocyanate, 1,5-hexane diisocyanate and 1,6-hexane diisocyanate, but araliphatic and aromatic diisocyanates are also suitable. Lactam-terminal polyisocyanates can be prepared by allowing a lactam, for example caprolactam, and a polyisocyanate to react. Preferably, blocked polymer HDI (hexamethylene diisocyanate) in caprolactam is used. The polyacyl-lactam compounds can be prepared by allowing a polyacyl compound, for example a polyacyl chloride compound, to react with lactam. Suitable polyacyl-lactam compounds are, for example, terephthaloylbiscaprolactam, adipoylbiscaprolactam, oxaloylbiscaprolactam, isophthaloylbiscaprolactum or mixtures of two or more of these compounds. Other suitable compounds supplying activator are triazines, carbodiimide and cyanamide. Instead of a homopolymer, nylon block copolymers can also be used in the method according to the invention. The nylon block copolymer, that in general consists of a polyamide segment and an elastomer segment, serves as prepolymer and can be reacted with lactam under the influence of the abovementioned catalyst and activator. Other copolymers which can be used are copolymers of a lactam and an elastomer. It is anticipated that such copolymers result in an improvement in the notch impact value. If desired, suitable additives which are customarily used in the preparation of nylon or nylon block copolymers are added during the preparation of polyamide mouldings. Examples of such additives are (non-chlorine-containing) flame extinguishers, colorants, impact strength improvers and stabilisers or adhesion improvers. Inter alia, organosilane compounds can be used as stabilisers in a concentration of, for example, 0.2 - 1 % (m/m). When producing polyamide mouldings containing mineral fibres, such as rock wool or glass wool, as filler, which will be explained below, it must be taken into account that the mineral fibres can already contain silane compounds since these are used as lubricants in the production process for the mineral fibres. The method according to the invention is intended for the production of polyamide mouldings having an optimum filler matrix, which is important, inter alia, in the production of mouldings which serve as a structural element. Structural elements in general consist of an inorganic or an organic filler which is held together by a binder. The filler is in general less expensive than the binder and therefore the filler is preferably used in a content that is as high as possible and with a selected grain distribution (matrix). However, mouldings are also provided for applications other than structural elements. In this context consideration can be given to mouldings for body shells of transport means, such as cars and aircraft. The strength of the material from which the moulding is composed is naturally of great importance for this purpose. A measure of the strength of a specific material can be obtained from the notched-bar (impact) test generally known to those skilled in the art in the field of material testing. A suitable filler material and/or binder material has to be selected on the basis of a specific application and the associated value required for the notch impact strength or notch impact work. A suitable guideline for carrying out a determination of the flexural strength of the material is testing in accordance with NEN 3835 from the Nederlands Normalisatie Instituut (Dutch Standardisation Institute). Inorganic and organic materials can be used as filler. Examples of inorganic materials are minerals, metals, metal oxides (Al 2 O 3 ), metal aluminates and metal silicates, silicon-containing materials, including quartz, sand, fly ash, marble and clay, as well as mixtures thereof. Sand or fly ash are fillers that are preferably used. A suitable organic material is, for example, carbon or graphite. Combinations of various fillers are also possible. A particular group of fillers is made up by fibres or fibre-containing materials. The filler content in, for example, a structural element is preferably more than 60 % (m/m). It is known from US 3 418 628 and NL 1 003 609 that a filler content of 95 % (m/m) constitutes an upper limit. The upper limit is formed by the quantity of polyamide that is needed to fill the volume between the filler particles. It is postulated that the demands on the filler particles in the case of a filler content greater than 95 % (m/m) become so extreme as to render an economically profitable process no longer possible. However, a filler content of 99 % (m m) is possible by the use of fibre or fibre-containing material as filler. The fibre structure has the advantage that even in the case of dense packing this still leaves sufficient space between the fibre particles, which can be filled by the lactam to be polymerised and the final polymer. Examples of inorganic fibre-containing materials which can be used are mineral fibres, such as glass wool and rock wool. Organic fibre- containing materials are, for example, dried manure, such as horse manure, and carbon fibre. In addition to the inorganic and organic fibres or fibre-containing materials, synthetic fibres are also known. Known synthetic fibres are aramide fibres, consisting of poly(p-phenyleneterephthalamide), or aromatic polyamide fibres. These are known for their exceptionally favourable strength/weight ratio. Especially in the case of mouldings which have to be lightweight and strong, such as, for example, in the case of the abovementioned body shells of cars and aircraft, the use of an aramide fibre as filler can be particularly advantageous. It is important that the filler is essentially moisture-free. The fillers therefore also have to be stored in a conditioned environment, dry and free from water. Any moisture present in the filler can have an adverse effect on the progress of the polymerisation of the lactam. As described above, the invention comprises feeding the composition which contains the monomer under pressure to a mould. With this procedure the mould is brought under reduced pressure prior to this step. According to a first aspect of the present invention, in addition to the composition which contains the monomer the filler is also fed under elevated pressure. According to a second aspect, at least some of the filler is present in the mould before the latter is brought under reduced pressure. The elevated pressure for feeding the composition which contains the monomer and the filler is preferably at least 2 atmospheres, more preferentially at least 4 atmospheres and most preferentially at least 6 atmospheres. In the case of larger mouldings this pressure can rise to even 10 or 15 atm. The elevated pressure under which the feed has to be supplied could be partly dependent on the viscosity and/or specific mass of the composition. What is the most suitable pressure for a specific composition will be determined experimentally depending on the composition and the dimensions of the article to be moulded. It will be clear to those skilled in the art, depending on the result of one or more experiments, whether a specific pressure is suitable or that an adjustment has to be made to the pressure under which filling is carried out. Although it is not restricted to the following range, it is to be expected that for the majority of processes the pressure under which a mould is filled can vary between 5 and 15 atm. The reduced pressure in the mould is preferably less than 0.5 atmosphere, more preferentially less than 0.2 atmosphere and most preferentially less than 0.1 atmosphere. In general the polymerisation of the monomer according to the invention will be initiated because the temperature of the components rises above the initiation temperature. The temperature of the components will be so controlled that the mixture of filler and composition which contains the monomer is at a temperature which is above the initiation temperature as soon as it is in the mould. Preferably, the mould is at a temperature which is equal to the temperature which the mixture has immediately after the components have been fed to the mould, which is a temperature that is above the initiation temperature. This leads to a homogeneous polymerisation process. With the method according to the invention it is therefore not necessary to allow the mould to cool below the initiation temperature between two production runs, as a result of which the method offers an economic advantage because it saves time and energy. The initiation temperature is dependent on the type and the quantity of the catalyst and activator which are used and is generally between 80 °C and 250 °C. More particularly the initiation temperature is between 130 °C and 150 °C. It is obvious that the temperature of the component which contains lactam, catalyst and activator must be below the initiation temperature since otherwise the polymerisation reaction already takes place before mixing with the filler. According to the first aspect of the invention, the method is carried out by bringing a closed, heated mould under reduced pressure and then filling this mould from a container which is connected to the mould and has been brought under elevated pressure. The container preferably contains a mixture that contains liquid lactam, a filler, catalyst and activator and the mixture is heated to a temperature that is above the initiation temperature. With the method according to the invention the composition which contains the monomer can be mixed with the filler before the mould is filled. It is also possible to feed the filler separately from the monomer composition. Which embodiment is preferred depends on, for example, the size of the mould and the reactivity of the activator and/or the catalyst and on the type of filler. This embodiment according to the invention consists in bringing a closed, heated mould under reduced pressure. The mould is then filled from at least two containers connected to the mould. The containers have been brought under elevated pressure. One container contains at least filler at a temperature which is above the initiation temperature. The other container contains a mixture of at least lactam, catalyst and activator and is at a temperature which is below the initiation temperature. Once again, as described above, those skilled in the art will be able to establish the pressure to be employed, depending on the size of the mould and the type of filler and the type of monomer and type of activator and type of catalyst and, where appropriate, yet further variable conditions. Although it is not restricted to the following range, it is to be anticipated that for the majority of processes the pressure under which a mould is filled can vary between 5 and 15 atm. A mould with containers for feeding materials to the mould connected thereto in several locations is shown diagrammatically in the figure. In the diagrammatic representation in the figure the feed is shown on each side of a rectangular mould from four containers. The position of the containers and the number of locations where containers are located, as well as the number of containers, can vary. It can also be envisaged that in the diagrammatic representation in the figure in the case of a specific production process filling of the mould does not take place from every container which is connected to the mould. It is shown in the figure that the mould can be heated. In addition, only the fill opening of the containers connected to the mould is shown. In this context it must be borne in mind that the containers themselves and also feed lines to the mould can be heated. Four sets of four fill openings are shown. In this context consideration can be given to a fill opening for the feed of monomer to be polymerised, a fill opening for the feed of filler, a fill opening for the feed of activator and a fill opening for the feed of catalyst. It is also possible to combine some of these constituents, such that not all fill openings have to be used. For example, monomer, catalyst and activator can be mixed and fed from one container, whilst filler is fed from a second container. It is important that with this embodiment the container containing monomer, catalyst and activator is at a temperature which is lower than the initiation temperature for the polymerisation reaction. It is also conceivable that yet further fill openings issue into the mould, it being possible for yet further components to be fed. Before filling the mould, the mould is brought under reduced pressure via valve PC A3. When the pressure has been sufficiently reduced, if possible preferably to virtual vacuum, valve PCA3 is closed. The mould is filled by opening valves PCA1 and PCA2. Homogeneous mixing in the mould takes place by eliminating the pressure difference between the mould and the containers from which the mould is filled. At the point in time when the mould has been filled it is advisable to maintain the fill pressure for some time, for a few seconds up to just before the point in time at which the mould is opened to release the moulding, depending on the mould. After releasing the moulding, the cycle can be repeated. All valves can be individually operated manually, but preferably opening and closing of the valves is carried out under computer control. For the sake of completeness it is pointed out that valves PCA1 and PCA2 for the four material streams indicated A, B, C, and D can be operated independently of one another, so that filling can take place from any container A, B, C or D or any combination of A, B, C and D. When filling the mould separately with the filler and the composition which contains the monomer the two components are preferably fed to the mould simultaneously or virtually simultaneously. Since the mould is under reduced pressure and the components to be fed are under elevated pressure, filling of the mould will take place very rapidly. In order to obtain good and homogeneous mixing, the feed of the one component will not be able to lag too far behind the feed of the other component. If the components are not fed simultaneously a number of factors are important with regard to how far the feed of the one component may lag behind when feeding the other component. Important factors are, for example, the pressure in the mould and the pressure on the components to be fed, the dimensions of the mould, the precise composition of the components and the temperature of the individual components and the mould and the type of filler. For example, the lower the pressure in the mould and the higher the pressure on the individual components the shorter will be the time that the one component may lag behind the other. Depending on the conditions as mentioned above, it will be clear to those skilled in the art to what extent the feed of the one component can lag behind the feed of the other component. As has already been pointed out above, a mixture to be polymerised becomes more viscous with a higher content of filler. In the case of the method according to the invention a high viscosity of the mixture is tolerated because the mixture is fed to the mould under elevated pressure. In the case of prior mixing of filler and the composition which contains the monomer and heating thereof to above the initiation temperature for the polymerisation, the polymerisation already starts before the mixture is in the mould. As a result only mouldings of limited size can be produced using the methods according to the prior art. However, in view of the high speed at which moulds can be filled according to the invention it is possible to produce much larger mouldings. The consequence of the elevated pressure on the viscous mixture to be fed to the mould, in combination with the reduced pressure in the mould, is that mouldings which have a filler content of more than 90 % can be produced. In the case of the second aspect according to the invention described above some of the filler is already present in the mould and the mould is brought under reduced pressure before filling and the composition which contains the monomer is fed under elevated pressure. Preferably, the remainder of the filler is also fed under elevated pressure. It is advantageous if at least 50 % (m/m) of the total quantity of filler is present in the mould. According to the prior art prefilling of a mould with filler will in general not lead to homogeneous mouldings with constant characteristics. Under the influence of gravity, for example, a filler such as sand will settle to the bottom of the mould and will not lead to a homogeneous mixture at the point in time when lactam is fed in. However, when a lightweight, fibre-containing material is used the filler will distribute over the entire available volume, especially in the case of high filler contents. When lactam, catalyst and activator are not under elevated pressure when fed to a mould in which filler is already present the lactam stream will be retarded by the filler. As a result an inhomogeneously polymerised moulding is obtained. When the method according to the second aspect of the invention is used a mould which already contains a filler is filled so rapidly that a homogeneous mixture and a homogeneously polymerised moulding are obtained. Preferably, according to the second aspect of the invention, mineral fibre, more preferentially rock wool or glass wool, is used as the filler which is introduced into the mould beforehand. In addition, it is also preferred to use synthetic fibres, such as, for example, aramide fibres, as filler. The method according to the second aspect of the invention can be carried out by placing mineral fibres, such as rock wool or glass wool, in the form of a sheet or loosely, in a mould and then bringing the closed, heated mould under reduced pressure. The temperature of the mould containing the mineral fibres is brought above the initiation temperature. The mould is then filled from a container which is connected to the mould and has been brought under elevated pressure. The container contains a mixture that contains at least lactam, catalyst and activator. In a comparable manner it is possible, for example, to process mats of, for example, aramide synthetic fibre into mouldings. If rock wool or glass wool is used as filler it is also possible by means of this invention to produce combinations between solid or fibre fills without any problem, depending on the characteristics required. The mouldings obtainable using the method according to the invention contain more than 50 % (m m), preferably more than 75 % (m/m), more preferentially more than 90 % (m/m) and most preferentially more than 95 % (m/m) filler. The mouldings obtainable using the method according to the invention are outstandingly suitable as structural elements. The cost price of structural elements having a high filler content is low, whilst with a high filler content a high modulus of the structural element is achieved. Structural elements according to the invention are, inter alia, suitable as a replacement for concrete. Because the binder content (polyamide) is low in the case of high filler contents, the water absorption by these structural elements is less than 4 %. Another advantage of these structural elements is that they can be recycled and are environmentally friendly. The mouldings containing aramide synthetic fibre are suitable for applications which impose specific requirements in respect of the combination of strength and weight of the moulding, such as, for example, in the case of body shells of aircraft and cars.

Claims: Claims 1. Method for the production of a polyamide moulding that contains a filler, wherein a composition which contains at least one monomer suitable for forming a polyamide is allowed to polymerise in a mould in the presence of the filler, characterised in that the composition is fed to the mould under elevated pressure and in that the mould is brought under reduced pressure before the composition is fed in. 2. Method according to Claim 1, wherein the filler is also fed to the mould under elevated pressure. 3. Method according to Claim 2, wherein the monomer is a lactam and the composition also contains an activator and catalyst. 4. Method according to Claim 2 or 3, wherein the filler is fed to the mould separately from the composition. 5. Method according to one of Claims 2 to 4, wherein the pressure in the mould is lower than 0.5 atmosphere, preferably is lower than 0.2 atmosphere and more preferentially is lower than 0.1 atmosphere before the composition and the filler are fed in. 6. Method according to one of Claims 2 to 5, wherein the elevated pressure for feeding the composition and the filler is higher than 2 atmospheres, preferably is higher than 4 atmospheres and more preferentially is higher than 6 atmospheres. 7. Method according to one of Claims 2 to 6, wherein the polymerisation of the monomer is initiated in that the temperature of the mixture of filler and composition is above the initiation temperature for polymerisation as soon as said mixture is in the mould. 8. Method according to Claim 7, wherein the temperature of the mould is above the initiation temperature. 9. Method according to Claims 4 and 7, wherein when filling the mould the filler is at a temperature which is above and the composition is at a temperature which is below the initiation temperature. 10. Method according to one of the preceding claims, wherein the filler is sand. 1 1. Method according to Claim 3, wherein the lactam is ε -caprolactam. 12. Method according to one of the preceding claims, wherein the polyamide is formed from a copolymer of lactam and an elastomer. 13. Polyamide moulding obtainable according to one of Claims 2 - 12. 14. Use of the polyamide moulding according to Claim 13 as a structural element. 15. Method according to Claim 1 or 12, wherein at least some of the filler is present in the mould before the latter is brought under reduced pressure. 16. Method according to Claim 15, wherein the remainder of the filler is fed under elevated pressure to the mould. 17. Method according to Claim 15 or 16, wherein at least 50 % (m/m) of the total quantity of filler is present in the mould. 18. Method according to one of Claims 15 to 17, characterised in that the monomer is a lactam and in that the composition also contains an activator and catalyst. 19. Method according to one of Claims 15 to 18, characterised in that the filler is fed to the mould separately from the composition. 20. Method according to one of Claims 15 to 19, wherein the pressure in the mould is lower than 0.5 atmosphere, preferably is lower than 0.2 atmosphere and more preferentially is lower than 0.1 atmosphere before the composition and the filler are fed in. 21. Method according to one of Claims 15 to 20, wherein the elevated pressure for feeding the composition and the filler is higher than 2 atmospheres, preferably is higher than 4 atmospheres and more preferentially is higher than 6 atmospheres. 22. Method according to one of Claims 15 to 21, wherein the polymerisation of the monomer is initiated in that the temperature of the mixture of filler and composition is above the initiation temperature for polymerisation as soon as said mixture is in the mould. 23. Method according to Claim 22, wherein the temperature of the mould is above the initiation temperature. 24. Method according to Claims 19 and 22, wherein when filling the mould the filler is at a temperature which is above and the composition is at a temperature which is below the initiation temperature for polymerisation. 25. Method according to one of Claims 15 to 24, wherein that portion of the filler that is present in the mould comprises mineral fibres, preferably rock wool. 26. Method according to one of Claims 15 to 24, wherein that portion of the filler that is present in the mould comprises synthetic fibres, preferably an aramide fibre. 27. Method according to Claim 18, wherein the lactam is ε -caprolactam. 28. Polyamide moulding obtainable according to one of Claims 15 to 27. 29. Use of the polyamide moulding according to Claim 28 as a body shell for transport means or as a structural element.

Inventors: TAAL LEENDERT, TAAL, LEENDERT
Applicants: MIJ VHB, TAAL LEENDERT, MAATSCHAP VHB, TAAL, LEENDERT
Application year: 2000
Main classification: C08K3
IPC codes: B29C45, B29K77, B29K105, B29L31, C04B26, C08G69