DE102008006588B4 - Process for the production of molded parts from thermosetting plastics - Google Patents

Abstract

A process for the production of molded parts from thermosetting plastics, characterized in that a forming process of a thermosetting plastic is carried out, characterized in that a) initially a thermosetting plastic is crosslinked so far that a thermosetting starting material is prepared, which is a partially crosslinked, gelled and thus infusible state b) and the starting product is thereafter reformed to its final shape by b1) being formed in a rubbery state in a range above the glass transition temperature (Tg) and below the decomposition temperature of the respective curing state, b2) and by cooling below the glass transition temperature Glass transition temperature (Tg) is prepared a glassy state of the converted starting product, wherein the conversion of the starting material is obtained, and c) that then the molded part by post-crosslinking in a largely completely crosslinked n state is transferred.

Description

The invention relates to a method for the production of molded parts from thermosetting plastics, in particular for the production of lightweight profiles for use in aerospace engineering, vehicle construction, mechanical engineering and mechatronics and the electronics industry.

During the deformation or deformation of thermoplastic materials, the thermoplastic is converted by heating into a softened material state in which it can be processed by forming technology and then reverted to a solid, stable state by cooling. The good and reversible formability of thermoplastics at relatively high temperatures is based on the increasing chain mobility of the macromolecules forming them and the lack of crosslinking of the macromolecule chains with one another.

From the publication US 4,332,767 A describes a conventional process for the production of molded parts from thermoplastic elastomers, in which thermoplastic intermediates are transformed and cured with only thermoplastic crosslinking by heating, melting and compression in their final state.

However, the advantageous production of molded parts made of thermoplastics and industrial applications are limited by the achievable mechanical, chemical and thermal properties of the thermoplastic material, which are much higher or better when using thermoset materials.

However, it is disadvantageous in the use of thermoset materials that they can only be prepared in a formable manner due to their irreversible chain crosslinking. This initial formation is usually done by mixing together reactive chemical components, which react with each other and form the macromolecule network. This crosslinking is irreversible and can only be reversed by thermal decomposition.

Since the crosslinking reaction is a time-consuming process, the economical and cost-effective production of thermoset molded parts are limited due to the high production cycle times. The complicated handling of the most liquid starting materials is disadvantageous.

In the field of unreinforced, short-fiber or particle-reinforced thermosetting plastics in particular two urformtechnische manufacturing processes for thermoset molded parts are known, the resin injection technique and the compression molding technology.

In the resin injection technique, the liquid reactive components are mixed together in a mixing device and then injected into the mold cavity. The mixing of the individual components can also take place within the mold cavity. To cure or carry out the crosslinking reaction of the components, the mixture is heated over a system-dependent consolidation time.

The compression molding technique is based on the use of specially adjusted resin compositions, which are doughy to firm, but informal at room temperature and can be converted by heating again in a liquid state. This informal resin composition is distributed by pressing force in the mold cavity and then thermally cured.

In the field of continuous fiber reinforced plastics, mainly the resin injection technique and the autoclave technique are used as well-known thermoforming methods. In the resin injection technique, the dry reinforcement fabric is cured by injection of the liquid resin matrix followed by thermal crosslinking to the composite.

In the autoclave technology, the starting material already comprises a resin impregnated textile, which is shaped in the autoclave by thermal treatment of the resin and pressing. The resin is thermoset crosslinked and cured.

From the publication EP 1 547 753 A1 Such a method for producing thermoset composite materials using an autoclave process is known. Prepegs impregnated with a thermosetting resin system are repeatedly heated and cooled to produce intermediates in a two-stage preforming process. The intermediates are thermoformed in the final autoclave process with complete thermosetting cross-linking and curing and at the same time connected with other moldings.

Known processes for the production of thermoset molded parts are furthermore the extrusion or extrusion process and the pultrusion process.

In all known methods, a resin system is used as the starting material, which is present in liquid or thickened form, while the urformtechnischen production is transferred to fill the mold cavity in a liquid form and then within the tool z. B. thermally networked, that is converted into a solid consolidated state. Fillers may already be embedded in the resin system or introduced into the mold cavity dry, the embedding is carried out only by infiltration with the liquid resin phase.

These manufacturing processes are purely forming processes, that is, from a formless material is created by creating a substance cohesion, a solid body, which represents the geometric and material Erstform a product.

A disadvantage of this production method is that the molding material must be converted into a liquid phase in order to fill up cavities in the molding material and to wet any fibers or particles present. The associated volume shrinkage must be compensated by complicated tool design.

Furthermore, the sealing problem of the tools as a result of the use of liquid, usually low-viscosity mold materials of disadvantage.

The consolidation of thermoset materials is also associated with a high expenditure of time, which has an effect on the cost of the profiles produced by the low tool throughput.

In the field of manufacturing processes of thermoset composite materials has the disadvantage that high demands on the process automation must be made because the reinforcing components do not have sufficient inherent rigidity and resistance to displacement to use conventional handling systems can.

A purely Umformtechnische production thermoset plastic molded parts is not yet known.

An approach to the forming technology processing thermosetting resin systems discloses the document EP 0 531 840 B1 , It is a process for producing a fiber-reinforced plastic molding in which the resin system of a fiber composite material is chemically modified so that it behaves thermoplastic up to a certain temperature, therefore re-meltable. For crosslinking, the resin system is soaked after forming with further reaction resin and heated strongly, so that the resin system hardens.

A disadvantage of this solution is that the preformed, thermoplastic behaving precursor loses its shape during the consolidation by the strong heating of the resin system and melts. Since the curing of the thermosetting portion requires a certain amount of time, in which the thermoplastic part is already completely softened or melted, the curing must be carried out in a tool which supports the softened precursor until complete crosslinking.

It is therefore an object of the invention to provide a method for the production of molded parts from thermoset materials to achieve an economically efficient processing unreinforced or reinforced thermosetting plastics in any geometries having moldings and to suppress the disadvantages described largely.

• a) zunächst ein duroplastisch vernetzender Kunststoff soweit vernetzt wird, dass ein duroplastisches Ausgangsprodukt hergestellt ist, welches einen teilvernetzten, gelierten und damit unschmelzbaren Zustand aufweist,
• b) und das Ausgangsprodukt danach dadurch in seine Endform umgeformt wird, dass
• b₁) es in einem gummielastischen Zustand in einem Bereich oberhalb der Glasübergangstemperatur (Tg) und unterhalb der Zersetzungstemperatur des jeweils vorliegenden Aushärtezustandes geformt wird,
• b₂) und durch Abkühlen unterhalb der Glasübergangstemperatur (Tg) ein glasartiger Zustand des umgeformten Ausgangsproduktes hergestellt wird, wobei die Umformung des Ausgangsproduktes erhalten wird, und
• c) dass anschließend das Formteil durch Nachvernetzung in einen weitestgehend vollständig vernetzten Zustand überführt wird.

The object of the invention is achieved in that a forming process of a thermosetting plastic is carried out, characterized in that

• a) initially a thermosetting plastic is crosslinked so far that a thermosetting starting material is prepared, which has a partially crosslinked, gelled and thus infusible state,
• b) and the starting material is thereafter reformed into its final form, that
• b ₁ ) it is molded in a rubber-elastic state in a range above the glass transition temperature (Tg) and below the decomposition temperature of the respective curing state,
• b ₂ ) and by cooling below the glass transition temperature (Tg) a glassy state of the converted starting product is produced, wherein the conversion of the starting material is obtained, and
• c) that then the molding is transferred by post-crosslinking in a largely completely crosslinked state.

With this method according to the invention molded parts of arbitrary geometry can be produced in a time-saving and cost-saving manner from thermosetting plastics, as it were in the forming process. Forming and curing process are separated, so that a fast forming process with correspondingly short cycle times is achieved.

The tools required can be utilized to a maximum, due to the relatively low forming forces occurring as a result of the softened forming state of the starting products, the tool stress and thus also the tool costs are reduced.

The solid, intrinsically stable starting products allow for uncomplicated handling Forming process and a high degree of automation of the procedure.

The fact that no melting of the starting product takes place during the forming process, the structural design of the tools can be simplified and the dimensional accuracy of the molded parts is less problematic.

According to an advantageous embodiment of the method according to the invention, the starting product is a thermosetting crosslinking reaction resin system.

An advantageous embodiment of the method according to the invention provides that the thermosetting crosslinking reaction resin system is particle or fiber-reinforced.

A further advantageous embodiment of the method according to the invention provides that the molded part is kept below the intrinsic glass transition temperature during postcrosslinking.

This undesirable reversion of the formed parts are avoided. This happens because the post-crosslinking either by slow heating of the moldings below the intrinsic glass transition temperature of the respective degree of crosslinking or by other physical processes, eg. B. by radiation crosslinking.

It is also a post-crosslinking above the respective degree of crosslinking immanent glass transition temperature possible, but undesirable reverse deformation of the molded part may occur due to released residual stresses, so that during post-crosslinking support of the moldings is necessary to preserve the reshaped shape of the moldings.

With the method according to the invention, in particular in the case of reinforced thermosets, materials can be used in a variable manner and in a wide variety of plastic molding processes for the production of the starting materials in the forming process.

The invention will be explained in more detail with reference to an embodiment. The accompanying drawing shows in

1 a time-temperature transition diagram (TTT), which shows the degree of crosslinking of a thermosetting plastic as a function of the crosslinking or curing temperature and the crosslinking time for the preparation of the starting product, and

2 a forming scheme that illustrates the forming process of the starting material in the molding and its post-crosslinking, wherein the drawn line characterizes the degree of crosslinking, from which a melting of the thermosetting plastic is no longer possible when heated.

The forming process as described in the forming scheme 2 is based on the fact that even a gelled, thermoset crosslinked molecular network from a certain glass transition temperature Tg passes into a softened, rubber-elastic state in which a viscoelastic deformation of the reaction resin system is possible. The reformed state is obtained by cooling the reaction resin system below its glass transition temperature Tg. In this case, the viscous deformation component is kept tension-free, but the elastic component is only frozen by the lowering of the molecular chain mobility. The elastic component generates residual stresses, which would lead to a re-deformation of the component when the formed component is heated above or into the region of the glass transition temperature Tg.

As a starting material for the forming a partially crosslinked reaction resin system is used, which is so far crosslinked that it is already gelled and thus can not be remelted by heating - see 1 , Area 4 , The individual ranges numbered in the time-temperature transition diagram (TTT) are intended to illustrate the following states of the partially crosslinked reaction resin system:
1 - glassy, completely uncured
2 - liquid
3 - gel-like
4 - Glassy, but not completely cured
5 - glassy, fully cured
6 - gel-like, rubber-elastic
7 - degradation, charred

This partially crosslinked reaction resin system has the advantage that only the already crosslinked regions of the macromolecule must be softened and thus the softening of the overall component takes place more rapidly. In addition, better formability is achieved due to the uncrosslinked areas.

Furthermore, it is possible with partially crosslinked resins to crosslink the uncrosslinked portion of the resin only after the forming. For example, by slow heating below the intrinsic glass transition temperature Tg inherent in the respective crosslinking state, the further reaction of free reactants takes place, with the result that further crosslinking bridges are incorporated into the macromolecule network and this is further stiffened in its deformed geometry.

For the production of optimally formable starting products, the knowledge of the time- and edge condition-dependent crosslinking of the reaction resin system is an indispensable requirement. One possibility for this is that TTT diagram, since it shows many important time and temperature-dependent resin information, such as state of aggregation or gelling point of the resin, degree of crosslinking, glass transition temperature Tg, etc. It can be divided into areas with uniquely identified properties of the reaction resin system during its crosslinking. The starting material for the described forming process must have the properties described by the lower portion of the "glassy but not fully cured" range. This means that the reaction resin system is in the infusible, solid, glassy state, but still the largest possible proportion of the reaction resin system is uncrosslinked. The degree of crosslinking of the reaction resin should be so high that a transition to the liquid state of aggregation upon heating of the reaction resin is no longer possible. In this case, the lowest temperature at which the associated gelation is even possible, characterized by the gel temperature Tggel.

In an isothermal crosslinking of a reaction resin system below this temperature, it is not possible to put the reaction resin system in the gelled, ie infusible state. However, the isothermal crosslinking should advantageously not take place well above the gel temperature Tggel in order to preserve as many uncrosslinked constituents as possible.

Starting materials prepared in this way should be stored until the transformation below the crosslinking temperature or the gel temperature Tggel, so that the unwanted further crosslinking of the reaction resin system is prevented.

After forming takes place, as 2 illustrates, the further post-crosslinking or heat treatment by slow heating but always below the temporal intrinsic glass transition temperature Tg the respective crosslinking state.

The resulting glass transition temperature Tg of the reaction resin system to be tempered during annealing is sufficiently slow heating and below the maximum achievable resin-dependent glass transition temperature Tg∞ always above the respective annealing temperature. The difference between annealing temperature and correspondingly adjusted glass transition temperature Tg of the reaction resin system becomes ever smaller as the maximum glass transition temperature Tg∞ approaches. For the annealing of formed components, it is therefore advantageous to terminate the annealing process even before the maximum glass transition temperature Tg∞ in order to prevent softening of the reaction resin system and a risk of re-deformation associated therewith.

As reinforcement both a particle, short, long or continuous fiber reinforcement is possible. A combination of different unreinforced or reinforced plastics is also conceivable.

Claims (7)

A process for the production of molded parts from thermosetting plastics, characterized in that a forming process of a thermosetting plastic is carried out, characterized in that a) initially a thermosetting plastic is crosslinked so far that a thermosetting starting material is prepared, which is a partially crosslinked, gelled and thus infusible state , b) and the starting product is then characterized converted into its final form, that b ₁₎ it (in a rubbery elastic state in a region above the glass transition temperature Tg) and is shaped below the decomposition temperature of the respective present Aushärtezustandes, b ₂₎ and by cooling below the glass transition temperature (Tg) a glassy state of the converted starting product is produced, wherein the conversion of the starting material is obtained, and c) that then the molded part by post-crosslinking in a largely completely vern last state is transferred. A process for the production of moldings according to claim 1, characterized in that the starting product is a thermosetting crosslinking reaction resin system. A process for the production of moldings according to claim 2, characterized in that the thermosetting crosslinking reaction resin system is particle or fiber-reinforced. A process for the production of molded parts according to any one of claims 1 to 3, characterized in that the starting material is partially crosslinked in a near range above the gel temperature (Tggel). Process for the production of molded parts according to one of claims 1 to 4, characterized in that a starting product is produced, which has a glassy, but not fully cured state and is heated to reach the moldable rubbery state to a range above its glass transition temperature (Tg). A process for the production of molded parts according to any one of claims 1 to 5, characterized in that a stored after the partial cross-linking and before forming the starting material at a temperature below the gel temperature (Tggel) is maintained. Process for the production of molded parts according to one of claims 1 to 6, characterized in that the molded part is kept during the post-crosslinking below the intrinsic glass transition temperature of the respective degree of crosslinking.

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