FIELD OF THE INVENTION

The invention relates to the field of foam-molded rigid hard materials, in particular a process for the production of poly(meth)acrylimide materials.

BACKGROUND OF THE INVENTION

Poly(meth)acrylimide (PMI), is a polyimide serving as a precursor for the produc tion of poly(meth)acrylimide rigid foams. Due to their high stability paired with their low overall weight, these foams became especially prominent in aircraft and car construction as well as related technologies. Typically, poly(meth)acrylimide is either made from poly(meth)acrylate and a suit able amine, such as methylamine, or by thermal treatment of a (meth)acrylate and (meth)acrylonitrile copolymer. In any case, a PMI polymer block is subseguently grinded to provide a PMI granulate which is then foam-molded to produce the de sired PMI rigid foams. In certain procedures, the PMI granulate is prefoamed before the foam-molding. Thereafter, the thus obtained PMI materials or PMI cores are usually processed in a pressing tool to provide the desired composite parts. SUMMARY OF THE INVENTION

The production methods of PMI materials known in the state of the art suffer from various drawbacks. In order to avoid handling volatile and hazardous low molecular weight amines, the method of choice for producing poly(meth)acrylimide polymer or polymer blocks, is the copolymerization of (meth)acrylate and (meth)acryloni- trile and subseguent thermal treatment to cyclize the nitrile- and carboxy-function- alities under formation of the corresponding imides. Currently, this process is per formed as a two-step procedure. That is, in a first step, the copolymer is generated and in a second step, the solid polymer block undergoes thermal treatment. The thermal treatment of the respective copolymer block is often cumbersome and re- guires special apparatuses. For example, the copolymer block is sandwiched be tween two special glass plates, which have to be sealed at their sides. The temper ature is then provided by a water bath surrounding the sealed plates. Typically, it is rather difficult to achieve an uniform heat distribution within the copolymer. As a conseguence, this process is not only more expensive as it reguires a special imida- tion apparatus, but also because the imidation process itself is slow due to the non- uniform heat distribution within the polymer block.

Furthermore, for generating the PMI foams, at least four process steps are neces sary, which all reguire different tools and apparatuses: ( 1 ) Copolymer formation, (2) Imidation, (3 ) Grinding and (4) Foam-molding. Another drawback of currently employed technologies is that foam molding of PMI particles is rather difficult. In particular, parts comprising a complex geometry, such as small edges, channels, grooves, etc. often brake when the foam-molded PMI material is removed from the forming tool. Additionally, the surface of the obtained PMI foam materials is often structured or contains pores which has been found to exhibit deleterious effects on the surface guality of any corresponding composite materials.

Conseguently, despite its beneficial properties, PMI foams are still not as promi nently used as other rigid foams, such as polyvinylchloride or polyurethane. Rea- sons for this are, amongst others, the high costs and the difficulties associated with their production.

It is therefore an overall object of the present invention to improve the state of the art regarding processes for the production of poly(meth)acrylimide materials, thereby preferably avoiding disadvantages of the prior art fully or partly. In favorable embodiments, a cost efficient process for producing PMI materials is provided. In further favorable embodiments, the process reduces or minimizes the number of steps necessary to produce PMI materials, PMI form molded compo nents and/or PMI foams. In additional favorable embodiments, the process for producing PMI materials yields PMI foams or PMI foam-molded components, with an at least reduced num ber of pores, or at least pores with reduced pore sizes or with an essentially even surface. The overall objective is in a general way achieved by the subject-matter of the inde pendent claim. Further advantageous and exemplary embodiments follow from the dependent claims and the description.

According to a first aspect, the overall objective is achieved by a process for the production of poly(meth)acrylimide materials, wherein a granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile is prefoamed and imidated by thermal treatment in a single step to provide poly(meth)acrylimide particles. It has been surprisingly found that if a granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile is employed, prefoaming and imidation can be performed effi ciently in a single process step. In particular, the fact that a granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile is used significantly reduces the reac tion time reguired for the imidation. This represents a significant advantage over the currently employed methods, which usually reguire thermal treatment of sev eral hours up to several days. Furthermore, the process according to the invention allows to reduce the number of process steps to generate PMI materials, pre- foamed PMI particles or PMI molded foam components, as the imidation and the prefoaming can be concomitantly performed in a single step. Prefoaming and im idation may for example be performed in a pre-foaming unit. It is understood herein that the term (meth)acrylic acid includes methacrylic acid, acrylic acid, or mixtures thereof. The term (meth)acrylonitrile includes acrylonitrile, methacrylonitrile and mixtures thereof. The term poly(meth)acrylimide or the ab breviation PMI include both polyacrylimides and polymethacrylimides. In particular, these also include polymethacrylimides or polyacrylimides obtained from methac rylonitrile, acrylonitrile, methacrylic acid or acrylic acid, suitable derivatives thereof or mixtures therefrom.

Furthermore, it is clear to the skilled person that the terms "imidated" or "imidation" refer to imide formation. In the context of polymers, in particular in context of poly(meth)acrylimide, the skilled person understands that an imidation does not necessarily require that all nitrile and/or carboxy-functionalities within the polymer react to produce an imide.

Preferably, in all embodiments described herein, a granulated copolymer of (meth)acrylic acid and acrylonitrile may be employed, more preferably a copolymer of methacrylic acid and acrylonitrile.

In a preferred embodiment, the process for production of poly(meth)acrylimide materials comprises a preceding step, wherein the granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile is provided by grinding a copolymer block of (meth)acrylic acid and (meth)acrylonitrile. Typically, the grinding provides a granulate with a particle size of 0.2 to 4 mm, preferably 0.5 to 3 mm. In another embodiment, the thermal treatment is performed for 2 to 1 5 min, pref erably 3 to 7 min. The skilled person understands that the thermal treatment re quires thermal energy in a range which is suitable for prefoaming and imidation. Furthermore, it is clear to the skilled person that the required thermal energy is de- pendent on parameters such as particle size, degree of prefoaming, desired den sity, nature of the propellant, etc.

In a further embodiment, a core temperature of 1 70 to 250 °C of the granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile and/or the formed poly(meth)acrylate particles is reached during the thermal treatment. Preferably, the granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile is provided as a single layer during thermal treatment. Thereby, the heat can be uniformly distributed and uniform prefoaming is achieved.

In a preferred embodiment, the thermal treatment is performed by an infrared source or by an oven. Using an infrared source has the advantage that significantly high penetration depths can be achieved. Furthermore, it has been surprisingly found that it is at all possible to concomitantly prefoam and imidate a granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile. Thereby, I R radiation proved advantageous, as it allows to specifically define the energy applied to the granulated copolymer. Typically, the infrared source emits infrared radiation with wavelengths in the range of 1 to 1 0 pm. This range has been found to provide sufficient energy to allow pre foaming and imidation in a rapid and efficient way.

Alternatively, an oven, preferably a drying oven, may be used for performing the thermal treatment. It is for example possible to fill the oven with the granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile and expose the granulated copolymer to a stream of hot air. In particular, it may be possible to conduct a pre defined time/temperature program.

In another embodiment, the granulated copolymer of(meth)acrylic acid and (meth)acrylonitrile comprises a propellant. Typical propellants include for example low molecular alkanes such as pentane or hexane.

In another embodiment, the granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile further comprises crosslinkers and/or other acrylate-derived repeat units. In other words, the granulated copolymer of (meth)acrylic acid and (meth)acrylonitrile does not only comprise linear chains, but may also comprise crosslinked polymer chains. For example, allyl(meth)acrylate may be used as a crosslinker during the preparation of the copolymer of (meth)acrylic acid and (meth)acrylonitrile. Other suitable acrylate-derived repeat units are for example derived from methyl(meth)acrylate, ethyl(meth)acrylate or t-butyl(meth)acrylate. In a preferred embodiment, the poly(meth)acrylimide particles are foam-molded to provide a poly(meth)acrylimide molded foam component. Typically, the foam molding comprises the following steps:

Optionally coating the poly(meth)acrylimide particles with an adhesion promoter;

Filling the optionally coated poly(meth)acrylimide particles in a forming 5 tool;

Heating the forming tool to a forming temperature and pressurizing the forming tool to a forming pressure;

Cooling the forming tool to a cooling temperature to provide a molded form component;

o - Deforming the molded foam component.

It is clear to the skilled person that a forming temperature and a forming pressure refers to a temperature, respectively pressure range, which is suitable for foam molding. Furthermore, the term "forming temperature" refers to the temperature of the forming tool. 5 Preferably, the forming temperature and the forming pressure are maintained for a particular forming time.

The optional step of coating the poly(meth)acrylimide particles with an adhesion promoter can alternatively be performed before the foam molding. For example, the coating may be performed after prefoaming and imidation of the granulated0 copolymer of (meth)acrylic acid and (meth)acrylonitrile. In a preferred embodiment, the optionally coated poly(meth)acrylimide particles are preheated to a pre-foaming temperature, which is slightly below their softening temperature or the forming temperature. Typically, the optionally coated poly(meth)acrylimide particles are preheated before being filled in the forming tool, which is preferably also preheated. As a result, the overall process time can be significantly reduced. For example, if the forming temperature is 1 80 °C, the pre foaming temperature to which the PMI particles are preheated may be 1 50 to 1 70 °C, preferably 1 60 °C. Therefore, the pre-foaming temperature may typically be 1 0 to 30 °C, preferably 20 °C, below the softening temperature or forming tempera- ture.

In a further embodiment, the bulk density of the poly(meth)acrylimide particles is egual to the density of the molded foam component. This has the advantage that the cavity of the forming tool may be completely filled. As a conseguence, the foam molded component is evenly foamed over its complete geometry. In typical embodiments, the bulk density may be in the range of 40 to 400 kg/m ³ , preferably 70 to 1 50 kg/m ³ .

In a preferred embodiment, the optionally coated poly(meth)acrylimide particles are filled in an expandable bag prior to filling the particles in the forming tool. The expandable bag may for example be made from nylon, polyamides, polyesters or any other expendable polymer or suitable material, preferably nylon. The bag may further be a fleece, a non-woven, or a foil. Using such a bag has several advantages. Firstly, the bag can be readily and specifically prepared for each molded foam com ponent. For example, the bag can already contain a predetermined amount of op tionally coated poly(meth)acrylimide particles, which is specifically reguired for producing a particular molded foam component. Secondly, the weighing does nei- 5 ther have to be performed at the forming tool itself nor by the person operating the forming tool. Thus, the overall process is rendered more efficient. Thirdly, the sur face guality of the obtained poly(meth)acrylimide materials is significantly in creased.

In a further embodiment, the expendable bag is provided with at least partial in-0 herent stability and/or a predefined geometry. Such an embodiment has the ad vantage, that the optionally coated poly(meth)acrylimide particles can be readily prearranged and distributed within the forming tool as desired. For example, it may be possible to provide specific areas with higher or lower amounts of optionally coated poly(meth)acrylimide particles and/or areas with optionally coated5 poly(meth)acrylimide particles having a bigger particle size than in other areas.

Such a difference in size distribution within the forming tool has been shown to be beneficial for forming edges and angles. It is clear that such a difference in size dis tribution is possible in all embodiments of the invention, which include a foam molding step. However, using a bag simplifies the arrangement of the particles. 0 In another embodiment, the forming temperature is in the range of 1 80 to 260 °C, preferably 200 to 240 °C. The forming pressure may be in the range of 4 to 1 0 bar, preferably 5 to 6 bar. In yet another embodiment, the forming temperature is reached after 5 to 40 min, preferably after 1 0 to 20 min.

In this and in other embodiments, the forming tool may additionally be preheated to 1 20 to 250 °C before the optionally coated poly(meth)acrylimide particles are filled into the forming tool.

In preferred embodiments, the forming temperature and/or the forming pressure is maintained for 5 to 40 min. However, it is clear to the skilled person that the reguired time period depends on the geometry and /or the thickness of the desired PMI material and may be adjusted to a suitable period. In a preferred embodiment, the foam molding is essentially performed as an iso thermal process. Thus, after filling the optionally coated poly(meth)acrylimide par ticles in the forming tool, the forming temperature is maintained constant. Prefer ably, the forming temperature is maintained constant at a value in the range of 1 80 to 260 °C, preferably 200 to 240 °C, more preferably at 21 0 °C. It is noted that the skilled person understands that the term "essentially" in this context refers to the fact that in some embodiments, a certain heating time is reguired until the forming temperature is reached and remains constant.

In preferred embodiments, heating the forming tool is performed via rapid heating methods such as induction or via surface-near heating with suitable liguids, for ex- ample oil or silicon oil. Such rapid heating methods are advantageous, as the time and cost reguired for reaching the forming temperature is significantly decreased. Furthermore, this method allows a convenient way of performing the foam mold ing as an essentially isothermal process.

In still another embodiment, the forming tool is coated with PTFE and/or talcum powder, before the optionally coated poly(meth)acrylimide particles are filled into the forming tool. In particular, it has been observed that the removal of the molded foam component after cooling from the forming tool, i.e. the deforming, proceeds much smoother as compared to an uncoated forming tool. This is especially the case if the forming tool comprises complex edges or grooves. Conseguently, it is thereby avoided that the molded foam component adheres to the tool and breaks upon removal. Alternatively, at least parts of the inner surface of the forming tool may be covered with a suitable foil, such as aluminum foil.

In a preferred embodiment, the forming tool is rapidly cooled in the cooling step by contacting the surface with a cooling liguid. Preferably, the temperature gradient between the cooling liguid and the forming tool is high. In certain examples, the temperature of the cooling liguid may be in the range of -20 to 40 °C, preferably 5 to 20 °C. Thus, the temperature gradient may be in the range of 1 40 to 280 °C. Cooling may for example be achieved by dipping the forming tool in a bath con taining the cooling liguid, by positioning the forming tool under a stream of cooling liguid or by rapid introduction of the cooling liguid in a cooling chamber surround- ing the forming tool. In certain embodiments the forming tool may be removable and thus configured to be transported to the cooling liguid. The forming tool may also be positioned between cooling plates. The cooling plates may typically be maintained at a temperature of -20 to 40 °C, preferably 5 to 40 °C, more prefer ably 1 0 to 1 5 °C. The cooling liquid can be any suitable liquid, such as water, oil or silicon oil.

It is understood, that such a rapid cooling step is much faster than allowing the forming tool to cool by itself or by cooling with cold air. For example, cooling the forming tool by itself may require between 60 to 1 20 min, while rapid cooling may only require seconds to a few minutes. It has been surprisingly found that the rapid cooling entails not only a significantly reduced overall process duration, but also results in a molded foam component with a high-quality closed surface, preferably not comprising any pores.

According to a second aspect of the invention, the overall objective technical prob lem is achieved by a process for the production of a poly(meth)acrylimide molded foam component comprising the steps:

Providing prefoamed poly(meth)acrylimide particles;

- Optionally coating the poly(meth)acrylimide particles with an adhesion promoter;

Filling the optionally coated poly(meth)acrylimide particles in a forming tool;

Heating the forming tool to a forming temperature and pressurizing the forming tool to a forming pressure; Cooling the forming tool to a cooling temperature to provide a molded form component;

Deforming the molded foam component.

The prefoamed poly(meth)acrylimide particles may either be provided by any of the embodiments described above in the first aspect of the invention, or by any other method known in the prior art, for example by grinding a copolymer block of poly(meth)acrylimide and subseguent prefoaming of the grinded poly(meth)acrylimide particles.

The embodiments described above for the first aspect of the invention, which in particular relate to the foam molding are generally also applicable alone or in all possible combinations in the process according to the second aspect of the inven- tion.

EXAMPLES

Example 1

In a first step, a copolymer block of (meth)acrylic acid and (meth)acrylonitrile is grinded to provide the granulated copolymer of (meth)acrylic acid and (meth)ac- rylonitrile with an average particle size of 1 to 4 mm. The granulated copolymer is then prefoamed and imidated by thermal treatment such that a core temperature of 1 70 to 250 °C of the granulated copolymer of (meth)acrylic acid and (meth)ac- rylonitrile and/or the formed poly(meth)acrylate particles is reached, thus provid ing poly(meth)acrylimide particles. Example 2

The poly(meth)acrylimide particles of Example 1 are then coated with an adhesion promoter, and filled in the forming tool, which has been coated with PTFE and tal cum powder and has been preheated to 1 30 °C. The forming tool is closed, heated to the forming temperature of 250 °C and pressurized to the forming pressure of 5.5 bar. After 30 min, the forming temperature has been reached and the forming tool has been allowed to cool by itself to 80 °C.

Example 3a

The poly(meth)acrylimide particles of Example 1 are coated with an adhesion pro moter, and filled into the forming tool. The forming tool is closed and readily heated to the forming temperature of 21 0 °C and pressurized to 6 bar. The forming tem perature was maintained for 20 min. The forming tool was then allowed to cool by itself to 80 °C over 90 min.

Example 3b

5 The poly(meth)acrylimide particles of Example 1 are coated with an adhesion pro moter, and filled into the forming tool. The forming tool is closed and readily heated to the forming temperature of 21 0 °C and pressurized to 6 bar. The forming tem perature was maintained for 20 min. Subseguently, the forming tool was removed and rapidly cooled under a stream of cooling water with a temperature of 5 to 200 °C .

While the molded foam component obtained from Example 3a showed the occur rence of pores along with a partially structured surface, the molded foam compo nent of Example 3b displayed a smooth and even surface, without any noticeable pores.