HK1080099B - Surface-compacted foam - Google Patents

Description

Foam with compacted surface

Technical Field

The invention relates to a method for producing a foam having a compacted surface, to a foam having a compacted surface and to the use of the foam according to the invention.

Prior Art

DE 19925787 describes a process for preparing a polycarbonate from ROHACELL by lamination with a cover layer^®A method for producing a speaker membrane of the prepared foam type. The cover layer is used to increase strength. The lamination is carried out in a press at a temperature of more than 160 ℃ and at a pressure of > 0.4 MPa. The mechanical properties of the individual plastic foam moldings are not described at all with respect to the absence of a cover layer laminated thereon.

DE2147528 describes a molded body with a porous cross section and an integrally closed skin layer, which is formed by applying heat and pressure to an emulsion polymerization product formed from acrylates and methacrylates and vinyl acetate. The occlusive cortex may also be provided with decorative ornamentation.

DE2229465 describes a porous molded body with an integral transparent window. The emulsion polymerization product resulting from the acrylate and methacrylate esters is charged into a suitable mold and shaped into a transparent plastic at the desired location by pressing between two heated platens.

In both cases, smooth surfaces with high gloss are obtained.

EP272359 describes a process for producing composite bodies, in which a foam core made from PVC or PU is laminated and installed in a closed mold, which corresponds to the composite body to be produced. The expansion pressure of the foam is used to produce a composite structure between the foam and the laminate. Purpose(s) to

From ROHACELL^®Plastic foam bodies produced are known and are known from R ö hm GmbH&Kg. They are used in slave ROHACELL^®The prepared core and cover produce parts. As covering layer any known planar body can be used which is stable under the processing parameters, such as pressure and temperature, required for the production of the end product. These forms include, for example, films or foils comprising polypropylene, polyester, polyamide, polyurethane, polyvinyl chloride, polymethyl (meth) acrylate, and/or metals such as aluminum, among others. It is also preferred to use a mat or mesh (Bahnen) comprising: glass fibers, carbon fibers and/or aramid fibers. A mesh having a multilayer structure may also be used as the cover layer.

For example, a prepreg can be preferably used. These prepregs are webs preimpregnated with curable plastics, mainly glass fibre mats or glass filament fabrics, which can be processed into mouldings or semi-finished products by hot pressing. These are in particular the so-called GMT and SMC. In all wet lamination and wet impregnation methods, such as VARI (vacuum assisted resin infusion), RTM (resin transfer moulding), VARTM (vacuum assisted resin transfer moulding), RLI (resin liquid infusion), SCRIMP (Seemann composite resin infusion moulding), DPRTM (differential pressure resin transfer moulding) or SLI (single file infusion), the liquid resin wets the surface of the foamed plastic body and superficially penetrates into the pores of the foamed plastic body, which results in resin loss and additional weight.

Plastics reinforced with carbon fibres are also known, which are particularly suitable as covering layers.

The thickness of the cover layer is preferably from 0.05 to 10mm, preferably from 0.1 to 5mm and very particularly preferably from 0.5 to 2 mm.

To improve adhesion, adhesives may also be used.

The amount of adhesive to be applied is a problem. In typical applications, the amount of adhesive is about 500g/m of the joint surface². For applications where weight is critical, this is a problem because a portion of the adhesive penetrates into the foam cells and is not available for use in forming the adhesive layer.

The solution to this problem has hitherto been to smooth the foam surface with a light, thin knife coating material in an operating step prior to adhesive application.

However, this method is disadvantageous in that it requires additional operation steps.

It is therefore an object of the present invention to provide a plastic foam body which has a reduced resin absorption at the same adhesion force or when the foam core is used as a "fly-away-tool".

Solution scheme

The foam body which is important for the process according to the invention consists of a poly (meth) acrylimide foam.

The writing (meth) acryloyl includes methacryloyl, acryloyl and mixtures thereof.

Poly (meth) acrylimide foams for the core layer of films comprise repeating units which can be represented by the general formula (I)

Wherein

R¹And R²The same or different is hydrogen or methyl, and

R³is hydrogen or an alkyl or aryl group containing up to 20 carbon atoms, of which hydrogen is preferred.

The units of the structure (I) preferably constitute more than 30% by weight, particularly preferably more than 50% by weight and very particularly preferably more than 80% by weight of the poly (meth) acrylimide foam.

The production of rigid poly (meth) acrylimide foams which can be used according to the invention is known, for example from GB patents 1078425 and 1045229, DE patent 1817156 (US patent 3627711) or DE patent 2726259 (US patent 4139685).

It is possible, for example, in particular to form units of the general structural formula (I) from adjacent (meth) acrylic acid and (meth) acrylonitrile units by means of a cycloisomerization reaction when heated to 150 ℃ and 250 ℃ (cf. DE-C1817156, DE-C2726259, EP-B146892). The precursors are generally produced by first polymerizing the monomers in the presence of free-radical initiators at low temperatures, for example from 30 to 60 ℃ and subsequently heating to from 60 to 120 ℃ and are then foamed by heating to about 180 ℃ and 250 ℃ from the blowing agents present (cf. EP-B356714).

For this purpose, for example, a copolymerization product comprising (meth) acrylic acid and (meth) acrylonitrile, preferably in a molar ratio of from 2: 3 to 3: 2, can be formed first.

These copolymerization products can additionally be used with other comonomers, such as esters of acrylic acid or methacrylic acid, in particular with lower alcohols having from 1 to 4 carbon atoms, styrene, maleic acid or its anhydride, itaconic acid or its anhydride, vinylpyrrolidone, vinyl chloride or vinylidene chloride. The comonomer content should not exceed 30% by weight, preferably 10% by weight, and the comonomer may not or only hardly be cyclized.

Other monomers which can advantageously be used in a likewise known manner are small amounts of crosslinking agents, such as allyl acrylate, allyl methacrylate, ethylene glycol diacrylate or dimethacrylate, or polyvalent metal salts of acrylic or methacrylic acid, such as magnesium methacrylate. The proportion may be, for example, from 0.005 to 5% by weight.

The precursor may additionally comprise conventional additives. These include, in particular, antistatics, antioxidants, mold-release agents, flame retardants, lubricants, dyes, flow improvers, fillers, light stabilizers, and organophosphorus compounds, such as phosphites or phosphonates, pigments, weathering stabilizers and plasticizers.

Polymerization initiators used include those customary for the polymerization of methacrylates, for example azo compounds, such as azobisisobutyronitrile, and also peroxides, such as dibenzoyl peroxide or dilauroyl peroxide, or also other peroxide compounds, such as tert-butyl peroctoate or perketal, and optionally redox initiators (cf. in this connection H.Rauch-Punticam, Th.V ö lker, Acryl-und Methylverbindungen [ acrylic and methacrylic compounds ], Springer, Heidelberg, 1967 or Kirk-Othmer, encyclopedia of chemical technology, Vol.1, pp.286 and later, John Wiley & Sons, N.Y., 1978). The polymerization initiators are preferably used in amounts of from 0.01 to 0.3% by weight, based on the starting materials. It is also possible to advantageously combine polymerization initiators which have different decomposition properties over time and temperature. Highly suitable is, for example, the simultaneous use of tert-butyl perpivalate, tert-butyl perbenzoate and tert-butyl per 2-ethylhexanoate.

To foam the copolymerization product during the conversion to the imide group-containing polymer, blowing agents are used in a known manner which form a gas phase by decomposition or evaporation at 150-250 ℃. Blowing agents containing amide structures, such as urea, monomethyl urea or N, N' -dimethyl urea, formamide or monomethyl formamide, release ammonia or amines upon decomposition, which can cause additional formation of imide groups. However, nitrogen-free blowing agents, such as formic acid, water or monohydric aliphatic alcohols having 3 to 8 carbon atoms, such as propanol, butanol, isobutanol, pentanol or hexanol, can also be used. Typical amounts of blowing agent used in the reaction batch are from about 0.5 to 8% by weight, based on the monomers used.

For example, polymethacrylimide foams which can very particularly preferably be used can be obtained by the following steps:

1. the polymer sheet is produced by free-radical polymerization in the presence of one or more initiators and optionally further conventional additives, which are listed above by way of example and consist of

(a) A monomer mixture consisting of 40 to 60 wt.% of methacrylonitrile, 60 to 40 wt.% of methacrylic acid and optionally up to 20 wt.%, based on the total amount of methacrylic acid and methacrylonitrile, of other monofunctional ethylenically unsaturated monomers,

(b)0.5 to 8% by weight of a blowing agent mixture consisting of formamide or monomethylformamide and monohydric aliphatic alcohols having 3 to 8 carbon atoms in the molecule,

(c) a crosslinker system consisting of (c.1) and (c.2),

(c.1)0.005 to 5% by weight of an ethylenically unsaturated compound capable of free-radical polymerization containing at least two double bonds in the molecule, and

(c.2) 1-5% by weight of magnesium oxide dissolved in the monomer mixture,

2. foaming the plate at the temperature of 200-260 ℃ to obtain the polymethacrylimide plate, and then

3. The heat treatment was carried out in two steps, wherein the first step consisted of 2-6 hours at 100-.

Polymethacrylimides having high thermal deformation stability can additionally be obtained by reacting polymethyl methacrylate or copolymers thereof with primary amines, which can likewise be used according to the invention. A wide variety of examples of similar imidizations of such polymers may be representatively mentioned: US4246374, EP216505a2, EP 860821. High heat distortion stability can be achieved here by using arylamines (JP05222119A2) or by using special comonomers (EP561230A2, EP577002A 1). However, all these reactions do not give a foam but a solid polymer, which must be foamed in a separate second step in order to obtain a foam. Techniques for this purpose are also known in the art.

Rigid poly (meth) acrylimide foams are also commercially available, an example being ROHACELL from R ö hmGmbH^®It may be provided in various densities and sizes.

The density of the poly (meth) acrylimide foam before compaction is preferably 20kg/cm³-180kg/cm³Particularly preferably 50 to 110kg/cm³。

The thickness of the foamed plastic body before compaction is from 1 to 1000mm, in particular from 5 to 500mm, and very particularly preferably from 10 to 300 mm.

In the subject aspect of the invention, the surface of the foam is successfully compacted by applying pressure and heat in a press.

This is generally achieved by a so-called hot-press moulding process. These processes are well known in the art and include specific embodiments such as twin belt extrusion molding, SMC extrusion molding, and GMT extrusion molding. The extrusion molding process preferably uses spacers, so-called baffles. These make it easy to adjust the desired degree of compaction of the core layer, but are not intended to limit the invention thereby.

The pressure to be applied in the press is about 30% of the static compressive strength of the foam. These data can be found in the corresponding ROHACELL^®Model number data sheet. Specifically, the pressure during the molding process of the present invention is 0.1MPa to 16MPa, preferably 0.1MPa to 1MPa, more preferably 1MPa to 7 MPa.

The molding temperature is 170 ℃ to 250 ℃, preferably 200 ℃ to 240 ℃, and more preferably 180 ℃ to 200 ℃. For example, the temperature of the press is 180 ℃ to 240 ℃. The extent of surface compaction may be determined by the duration of the heating process.

If the edge region of the plastic foam body is heated, a thin compacted zone is obtained.

If the entire plastic foam body is heated, complete compaction is obtained.

A smooth surface was obtained in each case. The amount of adhesive that must be applied is from about 500g/m²Reduced to less than 50g/m²。

By varying the pressure applied in the press and thus the subsequent different degrees of compaction of the surface of the plastic foam body, the resin absorption can be adjusted within wide limits, for example less than 500g/cm²Preferably 300g/m²To 100g/m²。

The foams of the present invention have higher stiffness at low weight compared to uncompacted foams. There is also an improvement in the impact behavior, i.e.a surface compression strength, determined in accordance with DIN 5342, of at least 0.4MPa, which is greater than that of the uncompacted foam.

The smooth surface of the inventive foam body makes it possible for the first time to use the inventive foam body as a sacrificial core in a fiber composite component.

The core cannot be removed or can only be removed with difficulty at present because of the porous structure of the foam surface, so that the core often remains in place. The foam of the present invention now allows the core to be removed from the interior of the finished laminate assembly.

The surface-compacted foam body according to the invention can also be used for the production of water, land, air vehicles and spacecraft and parts thereof. And thus the invention also provides waterborne, terrestrial, airborne vehicles and spacecraft and parts thereof produced using the surface compacted foam shapes of the invention.

The surface-compacted cellular plastic body according to the invention can also be used for producing sandwich structures in the manufacture of machines and for producing sandwich structures in the manufacture of sports equipment.

Production examples

Example 1

In-press ROHACELL^®Surface compaction

1. Heating press (approximate foaming temperature)

2. Charging with cold foam

3. Closing the press to generate contact pressure (0 to pressure to generate cold compaction, for ROHACELL^®Desirably about 30% of the compressive strength

4. By post-adjustment of the pressure and thus further closing, the force-controlled press compacts the heated surface. The cold inner region remains shape stable and is not compacted.

5. The desired final thickness (degree of formation) is dictated by the inserted shim

6. After the final thickness has been reached, the press and thus the ROHACELL are cooled^®. What is important here is that after removal the ROHACELL^®Is dimensionally stable.

Example 2

ROHACELL by post-expansion in a press^®Surface compaction

1. Using spacers and ROHACELL^®A cold or hot (above the foaming temperature) press is provided, wherein the thickness of both is about the same. The pressure must be adjusted in such a way that the press does not suffer from post-foaming ROHACELL^®The back pressure of the gasket to open or change the position of the gasket.

2. If it is desired to run the process with an initially cold press, the press is then heated above the foaming temperature.

3. If the desired surface compaction is achieved by post-expansion, the press is cooled in the closed state. What is important here is that after removal the ROHACELL^®Is dimensionally stable.

Example 3

ROHACELL by post-expansion in a mould^®Surface compaction

1. By ROHACELL^®A cold or hot (above the foaming temperature) mold is provided, with a thickness approximately corresponding to that of the mold cavity.

2. The mold is closed and, if a cold mold is used, heated to above the foaming temperature at this time.

3. If the desired degree of surface compaction is achieved by post-expansion, the mold must be cooled in the closed state. What is important here is that after removal the ROHACELL^®Is dimensionally stable.

Example 4

Subjecting ROHACELL to a reaction^®Continuous surface compaction

1. Continuous (near foaming temperature) heating of the ROHACELL with suitable heating means (heating plate, radiation source, microwave, hot air, hot rolls, etc.)^®。

2. Downstream, the ROHACELL is pressed by pressure (cold rolls, etc.) against the surface^®And (5) surface compaction. At the same time or downstream, the ROHACELL must be sufficiently cooled^®Until dimensional stability is achieved.

Claims (15)

1. A process for producing a surface-compacted foam, characterized in that the foam is obtained from a commercially available homogeneous foam molding of poly (meth) acrylimide foam by heating and applying pressure.

2. The process according to claim 1, wherein the forming temperature is from 170 ℃ to 250 ℃.

3. The process according to claim 1, wherein the forming temperature is from 200 ℃ to 240 ℃.

4. The process according to claim 1, wherein the shaping temperature is from 180 ℃ to 200 ℃.

5. The method according to claim 1, characterized in that the pressure during the forming process is 0.1MPa-16 MPa.

6. The method according to claim 1, characterized in that the pressure during the forming process is 0.1MPa to 1 MPa.

7. The method according to claim 1, characterized in that the pressure during the forming process is 1MPa to 7 MPa.

8. Surface-compacted foam molded body obtained by the process according to any of claims 1 to 4, characterized in that the resin pick-up is less than 500g/cm²And a surface compressive strength measured according to DIN 5342 of at least 0.4 MPa.

9. Use of the surface-compacted foam body according to claim 8 as a sacrificial core in a sandwich structure.

10. Use of the surface compacted foam body according to claim 8 for the production of water, land, air vehicles and space vehicles.

11. Waterborne, terrestrial, airborne vehicles and spacecraft, characterized in that it is produced using a surface compacted foam profile according to claim 8.

12. Use of the surface compacted foam body according to claim 8 for the production of parts for water, land, air vehicles and space vehicles.

13. Parts for the production of water, land, air vehicles and space vehicles, characterized in that they are produced from the foam according to claim 8.

14. Use of the surface-compacted foam body according to claim 8 for producing sandwich structures in mechanical production.

15. Use of the surface-compacted cellular plastic body according to claim 8 for producing sandwich structures in the production of sports equipment.