Process for producing foam molded parts with aesthetic surfaces by foam injection molding

Abstract

The invention relates to a process for producing a foam molded part, wherein a foamable material S comprising a thermoplastic polymer matrix M and at least one foaming agent F is foamed by foam injection molding. The polymer matrix M is preferably based on at least one thermoplastic styrene copolymer, such as ABS and ASA, and wherein the at least one foaming agent F is selected from chemical foaming agents, releasing carbon dioxide, and physical foaming agents, being carbon dioxide or nitrogen.

Claims

1. A process for producing a foam molded part by foam injection molding, which comprises the following steps: a) providing a melt of a foamable material S comprising a thermoplastic polymer matrix M and at least one foaming agent F selected from chemical foaming agents releasing carbon dioxide and physical foaming agents selected from nitrogen and carbon dioxide, wherein the thermoplastic polymer matrix M comprises at least one thermoplastic styrene copolymer P consisting of, based on P: 1.1 5-95% by weight of a copolymer A consisting of, based on A: 1.1.1 70-76% by weight, of one or more vinyl aromatic monomer(s) A1 selected from styrene and substituted styrenes, 1.1.2 24-30% by weight, of one or more vinyl cyanide monomer(s) A2 selected from acrylonitrile, methacrylonitrile, or mixtures thereof, and 1.1.3 0-6% by weight, of one or more unsaturated copolymerizable monomers A3, 1.2 5-60% by weight of a graft rubber B consisting of, based on B: 1.2.1 10-95% by weight of a graft base B1 comprising, based on B1: 1.2.1.1 80-100% by weight of one or more rubber type monomer(s) B11 selected from butadiene, isoprene, and C.sub.1-C.sub.10 alkyl acrylates, and 1.2.1.2 0-20% by weight of one or more polyfunctional crosslinking monomer(s) B12, and 1.2.2 5-90% by weight of a graft shell B2 comprising, based on B2: 1.2.2.1 75-85% by weight of one or more vinyl aromatic monomer(s) B21 selected from styrene and substituted styrenes, 1.2.2.2 15-25% by weight of one or more vinyl cyanide monomer(s) B22 selected from acrylonitrile, methacrylonitrile, or mixtures thereof, and 1.2.2.3 0-10% by weight of one or more unsaturated copolymerizable monomers B23; b) filling a mold cavity K with the melt of the foamable material S by injection; c) foaming the foamable material S in the mold cavity K by reduction of pressure and/or increase in temperature; d) taking out the foam molded part; and e) optionally further working of the foam molded part; wherein the melt temperature shortly before or during injection of the melt into the mold cavity in step b) is in the range of 255° C. to 300° C., and wherein the mold temperature in step b) is in the range of 50° C. to 150° C., and wherein filling of the mold cavity K with the melt of the foamable material S by injection in step b) is carried out with an injection speed in the range of 100 to 200 ccm/s.

2. The process according to claim 1, wherein the foamable material S comprises as further additive one or more additives selected from fillers, plasticizers, impact modifiers, dyes, pigments, colorants, flame retardants, antistatic agents, mold-release agents, antioxidants, stabilizers, and lubricants.

3. The process according to claim 1, wherein the vinyl aromatic monomer(s) A1 and/or B21 are selected from styrene, substituted styrenes of the formula I: ##STR00002## wherein: R is C.sub.1-C.sub.8-alkyl or hydrogen, R.sup.1 is C.sub.1-C.sub.8-alkyl or hydrogen, with the provision that not both R and R.sup.1 are hydrogen, and n is 1, 2, or 3, and mixtures of these compounds.

4. The process according to claim 1, wherein the thermoplastic polymer matrix M consists of a blend of at least one thermoplastic styrene copolymer P and at least one further polymer selected from polycarbonates and polyamides.

5. The process according to claim 1, wherein the foamable material S comprises at least 0.1% by weight, based on the foamable material S, of a chemical foaming agent comprising bicarbonate and citric acid as sole foaming agent.

6. The process according to claim 1, wherein the melt temperature shortly before or during injection of the melt into the mold cavity in step b) is equal or higher than 270° C., and wherein the mold temperature in step b) is equal or higher than 70° C.

7. The process according to claim 1, wherein the melt temperature shortly before or during injection of the melt into the mold cavity in step b) is in the range of 265° C. to 290° C., and wherein the mold temperature in step b) is in the range of 50° C. to 100° C.

8. The process according to claim 1, wherein filing of the mold cavity K with the melt of the foamable material S by injection in step b) is carried out with an injection speed in the range of 150 to 180 ccm/s.

9. The process according to claim 1, wherein the mold cavity K exhibits a polished surface or a grained surface having grain domains of at least 10 μm.

10. The process according to claim 1, wherein the thermoplastic polymer matrix M consists of at least one polymer selected from acrylonitrile-butadiene-styrene copolymers, acrylonitrile-styrene-acrylate copolymers, and blends of said styrene copolymers with at least one polyamide.

11. The process according to claim 1, wherein the foaming in step c) is carried out by reduction of pressure and wherein the reduction of pressure and the foaming is performed together with the injection step b).

12. The process according to claim 1, wherein the foaming of the foamable material S in step c) results in a density reduction in the range of 5 to 20%.

13. The process according to claim 1, wherein the thermoplastic polymer matrix M consists of acrylonitrile-butadiene-styrene copolymers, acrylonitrile-styrene-acrylate copolymers, or blends of acrylonitrile-butadiene-styrene copolymer and at least one polyamide, wherein the melt temperature shortly before or during injection of the melt into the mold cavity in step b) is in the range of 265° C. to 290° C.; wherein the mold temperature in step b) is in the range of 70 to 90° C.; and wherein the foaming of the foamable material S in step c) results in a density reduction in the range of 7 to 18%.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1a shows the results of statistical data analysis of the experimental results concerning the thickness of the skin layer d.sub.SL in dependency from the injection speed v.sub.inj and the melt temperature T.sub.m. FIG. 1b shows the results of statistical data analysis of the experimental results concerning standard density reduction SDR in dependency from the injection speed v.sub.inj and the melt temperature T.sub.m. The results are obtained from statistical data analysis program MODDE using Novodur® HH106 (ABS) as polymer P.

(2) FIG. 2 is a diagram showing the standard density reduction SDR (the upper 3 curves) and the thickness of skin layer d.sub.SL (the lower 3 curves) in dependency from the injection speed v.sub.inj and the melt temperature T.sub.m. The results are obtained from statistical data analysis program MODDE using Terblend® N NM 21 EF (ABS/PA blend) as polymer P.

(3) The present invention is further illustrated by the following experiments and claims.

EXPERIMENTAL EXAMPLES

Example 1: Process for Producing Foams

(4) Hydrocerol® 473 (Clariant, Frankfurt), which is a chemical foaming mixture based on sodium bicarbonate and citric acid, was used as foaming agent F. The chemical foaming agent was used in an amount of 2% by weight, based on the total foamable composition.

(5) Further, a foam injection molding process performed with MuCell® process using nitrogen in an amount of 0.3%, based on the total foamable material, was carried out.

(6) The following styrene copolymers and blends thereof, are investigated in the testing: P1 Terblend® N NM 21 EF (INEOS Styrolution Group GmbH, Frankfurt), which is a blend of acrylonitrile/butadiene/styrene-copolymer and polyamide (ABS/PA); P2 Terblend® N NG02EF (INEOS Styrolution Group GmbH), which is a glass fiber reinforced blend of acrylonitrile/butadiene/styrene-copolymer and polyamide (ABS/PA) including 8 wt.-% of glass fiber; P3 Novodur® HH106 (INEOS Styrolution Group GmbH), which is an acrylonitrile/butadiene/styrene-copolymer (ABS); P4 Fibremod™ GE277A1 (Borealis), which is a commercial glass fiber reinforced polypropylene PP including 20 wt.-% filler (comparative example).

(7) The results were compared to foams made of commercially available polypropylene PP and glass-fiber reinforced polypropylene PP-GF (P4) as thermoplastic polymer matrix M, which are common commercial foam products and which were used as a benchmark.

(8) Also experiments were carried out using Luran® S, which is an acrylonitrile/styrene/acrylate-copolymer ASA; commercially available (INEOS Styrolution, Frankfurt). The molding tool was a 2K-plate with polished surface and 2.5 mmm wall thickness (400 mm×200 mm×2.5 mm). Further different molds with grained surface and 3 mm wall thickness were used (see example 1.3). An injection foam molding machine Engel Duo 450 with a 3-zone extruder of 60 mm was used.

(9) The mold temperature was 60° C.

Example 2: Variation of Foaming Conditions

(10) The standard density reduction before and after the foaming was determined using a hydrostatic balance.

(11) Standard reduction of density (SRD) in [%] is defined as: SRD=(d.sub.foam−d.sub.comp)/d.sub.comp*100%; with d.sub.foam is density after foaming and d.sub.comp is density before forming (compact solid polymer).

(12) Further, the thickness of the skin layer of the foam molded part was determined using light microscopy. The foam molded parts obtained are integral foams composed of an outer dense layer (also referred to as skin layer) and a light, cellular core.

(13) Using a standard computer program for statistical test planning and data analysis (MODDE) an optimum of process conditions in view of maximal standard density reduction and minimal thickness of skin layer d.sub.SL are determined. A maximum density reduction of 17.5% was obtained at melt temperature T.sub.M in the range of 265° C. to 300° C. and, in particular of about 270° C., and an injection velocity in the range of 150 to 190 cm/s, in particular of 160 ccm/s (cm.sup.3 per second). An minimum thickness of skin layer of 0.9 mm was obtained.

(14) The results of statistical data analysis are shown in FIGS. 1a and 1b (for Novodur® HH106 (ABS) as polymer P) and FIG. 2 (Terblend® N NM 21 EF (ABS/PA blend) as polymer P).

(15) The optimal conditions determined by statistical data analysis are as follows: injection speed of 165 ccm/s, a melt temperature of 270° C., a mold temperature of 80° C., a density reduction of 17.5% and a skin layer thickness of 0.9 mm.

(16) The mold temperature was varied from 50° C. up to 110° C. (with static temperature control) and from 50 to 140° C. (dynamic temperature control).

Example 3: Mechanical and Surface Properties of Foams

(17) a) The following polymer materials P were used

(18) Foams were produced as described in example 2 using the polymer materials P1 to P4 wherein 2% by weight, based on the foamable composition S, of the foaming agent Hydrocerorwere used. The produced foam molded parts exhibited a thickness of 2.5 mm (mold with polished surface).

(19) b) The bending stress given in [MPa] of the moldings was determined according to DIN EN ISO 178. The perforation energy given in [J] was determined according to DIN EN ISO 6603-1178 at a temperature of 23° C. and a penetration energy of 200 kJ. The results are summarized in the following table 1:

(20) TABLE-US-00001 TABLE 1 Mechanical properties of foam molded parts bending perforation polymer stress [MPa] energy [J] density material compact compact reduction No. P reference foam reference foam [%] 1 P1 50 44 52,000 11,800 10   2 P2 75 53  8,400  5,000 10   3 P3 73 63 12,500  6,100  7.5 C1 P4 74 65  4,200  2,900 10  

(21) The compact reference is the starting polymer material not foamed.

(22) The investigation of mechanical properties of the foam clearly shows that the use of styrene polymers, such as ABS, has advantages over glass-fiber reinforced polypropylene (GF-PP), in particular in view of impact strength of the foams. In particular because of an higher energy absorption of the ABS materials in comparison to GF-PP better mechanical properties can be achieved using ABS or ABS blends.

(23) c) In a second series, the surface quality of styrene copolymer foams versus glass fiber reinforced polypropylene foams was assessed after performing foam injection molding trials as described above.

(24) The results are summarized in the following Table 2:

(25) TABLE-US-00002 TABLE 2 Evaluation of surface quality Mold Wall Polymer tem- thickness of Density material perature foam part Mold reduction Surface No. P [° C.] [mm] surface [%] quality 4 P3 120-60 2.5 Polished 10.5 Unstable veils 5 P1 120-60 2.5 Polished 13.7 Unstable veils 6 P1  70 3   Grained  8.0 Light silver veils 7 P1 120-60 3   Grained  8.0 Good surface 8 P2 110 3   Grained  7.2 Good surface 9 P2 120-60 3   Grained  7.2 Good surface

(26) The standard tool (mold) with 2.5 mm wall thickness and a polished surface was used for Terblend N NM21 EF and Novodur HH106. As the results were promising, but not acceptable for a ‘Class A’ surface on a polished part, another mold with a grained surface having four grains on one mold and 3 mm wall thickness has been used for the further samples as well as for ASA.

(27) Surprisingly it was found that good surface quality can be achieved for grained structures using ASA (Luran® S) along with approximate 10% density reduction and with ABS/PA (Terblend® N) along with approximate 8% density reduction.