EP0691996A1

EP0691996A1 - Polyamide resin composition

Info

Publication number: EP0691996A1
Application number: EP93909571A
Authority: EP
Inventors: Ryuichi Hayashi; Masahiro Nozaki
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1993-03-29
Filing date: 1993-03-29
Publication date: 1996-01-17
Also published as: WO1994022942A1; EP0691996A4

Abstract

A polyamide resin composition comprising 30-50 parts by weight of polyamide resin and 70-50 parts by weight of a combination of glass fibers and glass flakes and having a resin melt viscosity, when molding, of 50-200 Pascal seconds at a shear rate of 1,216 sec-1. The state of warpage of molded articles from the polyamide resin compositions of example 17 and control example 1 is shown in the accompanying diagram.

Description

TITLE

POLYAMIDE RESIN COMPOSITION BACKGROUND

The present invention relates to a polyamide resin composition comprising a polyamide resin and a reinforcing filler, such as glass fibers, glass flakes, or mineral fillers, said composition exhibiting good flow and moldability and providing a high precision molded article with a smooth surface and low warpage, while maintaining high stiffness and high strength. Fiber reinforcing agents, such as glass fibers, have heretofore been mixed with polyamide resins for reinforcing the resins.

Improving the mechanical properties, such as stiffness and strength, of molded articles obtained from such glass fiber-reinforced polyamide resin compositions requires mixing a large amount of the glass fibers into the polyamide resin. However, the conventional glass fiber-reinforced polyamide resin compositions have been deficient in that it has been found that increasing the proportion of the glass fibers adversely affected the flow of the resin compositions when they were molded, particularly when they were injection molded, which made it difficult to form a large-sized part or a part with rib structures. In other words, increasing the concentration of the glass fibers mixed with the polyamide resin deteriorated the flow of the resin composition, thereby resulting in a need for higher injection pressure for injection molding. Further, it also tended to provide unfilled thin wall sections in molded parts, which substantially degraded the injection moldability of the resin, particularly for large-sized parts or rib-structured parts. Increasing the concentration of glass fibers mixed into the polyamide resin also suffered from a problem of the glass fibers rising to the surface of the molded goods, thereby adversely affecting the surface appearance of the molded articles. In addition, molded articles with glass fibers mixed therein tended to suffer from a problem of waφage, which made it difficult to obtain a high dimensional precision for a large-sized molded article.

The present invention aims to provide a polyamide resin composition capable of being molded into a high precision molded article with good stiffness, good strength, and low warpage and further capable of being molded by a conventional injection molding machine into a molded article with complicated and fine parts, or into a large-sized part, as well as being capable of generating a molded article with a smooth surface appearance.

The above objectives were met by a polyamide resin composition comprising 30-50 parts by weight of a polyamide resin and 70-50 parts by weight of a combination of glass fibers and glass flakes or mineral filler, said resin composition having a melt viscosity when molding, at a shear rate of 1,216 sec-¹, of 50-200 Pascal seconds.

The above objectives were also met by a polyamide resin composition comprising 30-50 parts by weight of polyamide resin and 70-50 parts by weight of glass flakes, said resin composition having a melt viscosity when molding, at a shear rate of 1,216 sec^-1, of 50-200 Pascal seconds. The above objectives were also met by a polyamide resin composition comprising 40-50 parts by weight of polyamide resin and 60-50 parts by weight of a mineral filler, said resin composition having a melt viscosity when molding, at a shear rate of 1,216 sec"¹, of 50-200 Pascal seconds.

DETAILED DESCRIPTION OF THE INVENTION Improving the moldability of a polyamide resin composition requires increasing the flow of the polyamide resin composition. In general, on decreasing the molecular weight of the polyamide, the polyamide flow improves, but a low molecular weight polyamide gives a molded article with reduced mechanical properties. A polyamide resin composition filled with a reinforcing agent will show a flow which varies depending upon the type of reinforcing agent, so that regulating the polyamide resin molecular weight by itself is not necessarily able to provide an optimum flow. Flow can be altered, and improved, by decreasing the amount of the reinforcement agent mixed for improved moldability, but that approach by itself will decrease the strength of the molded article. As was discussed above, increasing the amount of the reinforcement agent to be mixed into the polyamide will adversely affect the surface appearance of the resultant molded articles. In the present invention, it was discovered that the measures to be regulated so as to strike a good balance between the moldability of the resin, and the strength and appearance of molded articles made from the polyamide resin composition, are as follows: (1) the mixing ratio of the polyamide resin and the reinforcing agent and (2) the melt viscosity of the polyamide resin composition at a shear rate of 1,216 sec^-1. Limiting the above two items to specific ranges, and further employing glass flakes or mineral fillers as reinforcement agents, can provide simultaneously the excellent moldability and high strength of molded articles together with a favorable surface appearance and low waφage, of said articles.

"Melt viscosity", as used herein means a viscosity measured under a shear rate of 1,216 sec^-1 at the resin temperature during the molding of an absolutely dry molded article obtained from the polyamide resin composition. The viscosity can be, for example, measured by a Kayness capillary viscometer. The resin temperature during molding is about 280°C for a composition using nylon 66 as a polyamide resin. A polyamide resin composition having a melt viscosity, when molding, of less than 50 Pascal seconds, at a shear rate of 1,216 sec^-1, is not preferred because of problematic moldability behavior. Such a resin composition is considered to have a low melt viscosity, which results in the formation of a burr or a dripping resin composition (the so-called "running nose phenomenon"). On the other hand, a melt viscosity during molding exceeding 200 Pascal seconds, at a shear rate of 1,216 sec-¹, is not preferred in that a resin composition such as those of this invention with at least 50% by weight of fillers, such as a glass flakes, mineral filler, or glass fibers, will have phenomena such as poor appearance due to the glass flakes, and the like, rising to the surface of the molded article, as well as requiring a high injection pressure because thin wall sections during injection molding tend to fail to be filled by the resin composition.

It is preferred to mold the polyamide resin composition of this invention by injection molding a preparation so as to exhibit a resin melt viscosity at a resin temperature during injection molding (normally 15-40°C higher than the melting point) of 50-200 Pascal seconds at a shear rate of 1,216 sec¹.

The polyamide resin composition of this invention comprises a matrix polyamide resin which is filled with certain reinforcing materials, such as glass fibers, glass flakes, or mineral fillers, singly or in combination. The type of reinforcing material used is selected depending upon the desired physical properties of the molded articles in terms of stiffness, strength, and the like, as well as the extent of allowable waφage of the molded articles. Although it depends upon the type of glass flakes or mineral fillers used, in general, the greater the amount of glass flakes or mineral fillers used for filling, the less waφage and the higher precision of the molded articles, while at the same time, these more favorable types are provided with a delustered surface appearance. On the other hand, stiffness, strength, and the like, tend to decrease when only glass fibers are used for filling to the same level. Therefore, with the polyamide resin compositions of this invention, by selecting the type of glass flakes and mineral fillers, and by adjusting the ratio of mixing with glass fibers, one can generate molded articles with the required physical properties, low waφage, and excellent surface appearance.

The amounts of the reinforcing materials mixed with the polyamide resin (i.e., glass fibers, glass flakes, and mineral fillers) are expressed on the basis of the total weight of the polyamide resin and these reinforcing materials, Specifically in terms of parts by weight per 100 parts by weight of the total weight of the polyamide resin and reinforcing material. If the reinforcing material is a combination of glass fibers and glass flakes or if it is a combination of glass fibers and mineral fillers, or if it uses only glass flakes, then 50-70 parts by weight of these reinforcing materials are mixed with the complementary amount of polyamide resin, 50-30 parts by weight. If mineral fillers are used by themselves, 50-60 parts by weight of the mineral fillers are mixed with a complementary amount of the polyamide resin, 50-40 parts by weight. If the amount of reinforcing material incoφorated is greater than that defined here, the resultant resin composition will have reduced flow and reduced moldability, such as processability, and the like, and at the same time will experience difficulty in achieving a uniform mixed dispersion state, as well as ending up with a deteriorated surface condition for the molded article. Decreasing the amount of reinforcing material to be mixed to less than the range defined herein, will result in the molded article having insufficient mechanical strength.

The polyamide resin used as a matrix in this invention is preferably a low viscosity-type. Specifically, the resin is a low viscosity-type polyamide having a melt viscosity of not more than 80 Pascal seconds when tested absolutely dry at an injection molding resin temperature (normally 15-40°C higher than the melting point) and at a rate of 1,216 sec¹. Such a low viscosity polyamide resin can be prepared by molecular weight control during polymerization, for example, by generating a low molecular weight polyamide by controlling water during polymerization or by blending a high molecular weight polyamide with a low molecular weight polyamide. The mixing may be achieved by mixing pellets or mixing in the molten state. A polyamide resin with lower melt viscosity will be able to wet the surface of the glass fibers, and the like, so that the resultant polyamide resin composition will show improved processability and moldability, as well as prevent the filled glass fibers, and the like, from rising to the surface of the molded articles.

By the term polyamide in this invention is meant a linear synthetic polymer having amide-linkages in the main chain obtained by a polycondensation reaction of a diamine and a dibasic acid, ring opening polymerization of lactam, or polycondensation of an amino carboxylic acid (for example, nylon 6, nylon 66, nylon 68, nylon 610, nylon 612, or the like) or nylon copolymers of these components, also including aromatic polyamides. The glass fibers used in this invention are those normally used as a reinforcing materials, including any shape, long and short fiber glasses. Depending upon the glass fiber length, compounding by an extruder will require considering the correct design for the screw or using a downstream system. Preferred is the use of short fiber glass of a chopped strand type. The glass flakes used in this invention are preferably about 1 um to 8 um thick and not more than 1700 um in particle size. It is preferred to give a suitable treatment on the surface of the glass flakes, such as a silane treatment, or the like, so as to increase the adhesion with the polyamide resin. The particle size and particle size distribution should be accommodated by controlling the process conditions in compounding by an extruder or considering the optimum screw design, and the like.

The mineral fillers used in this invention include, for example, an Si0₂-MgO type such as talc, an Si0₂-CaO type, such as wollastonite (calcium silicate), an Si0₂-Al₂0₃ type, such as kaolin, an Si0₂-Al₂0₃-K₂0 type, such as mica, as well as calcium carbonate, calcium sulfate, aluminum hydroxide, magnesium oxide, and the like. Preferably, talc, wollastonite, kaolin, mica, and the like are silane surface-treated. The mineral fillers are preferably kaolin clay having a particle size of about 0.3-20 um which are also surface-treated with an amino silane, and the like. Glass beads can also be used in a similar manner to the glass flakes or mineral fillers. The glass beads which can be used are spherical with a diameter of about 10-100 um including ordinary glass beads having a surface treatment or similar hollow glass beads, and the like.

The polyamide resin composition of this invention, in addition to the above polyamide resin, glass fibers, glass flakes, or mineral fillers, may optionally be mixed with one or more usual additives, such as stabilizers against oxidation, heat, and ultraviolet light degradation or inhibitors thereof; lubricants, and mold release agents; colorants, including dyes and pigments; nucleating agents; blowing agents; plasticizers; inorganic fillers; flame retarders; antistatic agents; and the like. EXAMPLES

The present invention is specifically described by the following examples that follow.

Examples 1-10 and Control Examples 1-4:

(Preparation and Evaluation Method for Samples Containing Mineral Fillers or Mineral Fillers and Glass Fibers)

Nylon 66 having a suitable molecular weight or a blend of nylon 66 and nylon 6 was melt mixed with commercial chopped strand short glass fibers, 10 microns in diameter and 3 mm long, as a glass fiber component, in a twin screw extruder having optimally designed screws. In order to bring the resin melt temperature to 280-300°C, the extruder barrel temperature and screw revolution rate were adjusted. In a similar manner, a silane-treated kaolin clay as a mineral filler component was melt mixed with nylon 66 or a blend of nylons 66 and 6.

The resultant two types of samples were mixed to bring the blending ratios of the mineral fillers and glass fibers to values as shown in Table 1 to generate resin pellets.

The resultant resin pellets were injection molded as described below in order to evaluate moldability and to measure these molded articles in terms of waφage, surface condition, and dynamic properties. These resin pellets were each dried in dry air at 80°C prior to molding.

Moldability was evaluated by injection molding the resin pellets into box-like parts having dimensions 45 cm x 19 cm x 8 cm. Moldability was evaluated in terms of the pressure needed for molding, the ease of filling into a mold, conditions of molding machine barrels, "running nose" phenomenon in the nozzle section, etc. to give a rating of excellent, good, fair, or poor. The surface state, such as the surface roughness and waφage, of these molded articles, was visually inspected and given one of the ratings mentioned above.

Large-sized sheet molded articles were tested for waφage by injection molding sample pellets from Examples 2, 9, and Control Example 1 into sheets having a ribbed structure, 400 mm x 100 mm x 10 mm, and allowing 24 hours after molding before measuring waφage. Waφage was rated as excellent, good, fair, or poor.

Mold shrinkage was determined from 3 inch x 5 inch x 1/8 inch sheets molded from the resin pellets.

The physical properties for the resin pellets was determined by injection molding, on a 6 oz. injection molding machine, the pellets into test pieces having dimensions 13 mm x 130 mm x 3.2 mm. The reaction time was 5-6 minutes, the barrel temperature was 270-280°C, and the nozzle temperature was 280-290°C. The mold temperature was about 90°C so as to carry out a molding cycle of 10/20 or 20/20 cycles (ram forward in seconds/retention in seconds). The resultant molded articles were tested for their physical properties immediately after molding, as well as at 150°C. Molding and testing was done in accordance with ASTM D638. The resin melt viscosity of the above molded articles (for measuring physical properties) was measured using a Toyo Seiki capillograph viscometer for an absolutely dry molded article (containing 0.1-0.15% water) at 280°C at a shear rate of 1,216 sec¹.

The results of the measurements are given in Table 1 and Figure 1.

Examples 11-18 and Control Examples 5 and 6:

(Preparation and Method of Evaluation of Examples Containing Glass

Flakes or Glass Flakes and Glass Fibers)

Nylon 66 having a suitable molecular weight and silane-treated glass flakes, 4 um thick and having a particle distribution of 45-1,700 um, were melt mixed in a twin screw extruder with optimally-designed screws to reach blend ratios of the glass flakes and glass fiber given in Table 2. The extruder barrel temperature and the number of screw revolutions were adjusted so that the resin melt temperature was 280-300°C. Resin pellets were formed from the extruded resin. Molded articles were injection molded in a manner similar to that of Example 1 from these resin pellets to measure moldability, the surface appearance, waφage, and physical properties of the molded articles. The samples of Examples 11, 12, 16, and 17 were molded into sheets with a ribbed structure 400 mm x 100 mm x 10 mm to measure their waφage. The results are summarized in Table 2 and Figures 2 and 3.

Table 1 - Examples 1-3

G: Good F: Fair Table 1 - Examples 4-6

G: Good E: Excellent Table 1 - Examples 7 and 8

E: Excellent Table 1 - Examples 9 and 10

G: Good E: Excellent Table 1 - Control Examples 1 and 2

F = Fair P = Poor Table 1 - Control Examples 3 and 4

G = Good P = Poor Table 2 - Examples 11-13

E = Excellent F = Fair G = Good Table 2 - Examples 14 and 15

E = Excellent G = Good Table 2 - Examples 16-18

E = Excellent F = Fair G = Good Table 2 - Control Examples 5 and 6

F = Fair G = Good P = Poor The above results from the Examples and Control Examples demonstrate the following:

First of all, in regard to the effect of mineral fillers, as well as mineral fillers and glass fibers, Examples 1-6 and Example 9 containing mineral fillers and glass fibers showed significantly higher flexural modulus, flexural strength, and tensile strength than Control Example 4. A comparison of Control Example 1 with Examples 1-9 shows that the use of glass fibers alone gave high stiffness and strength; however, there was some room for improvement in the areas of moldability, molded goods' surface appearance, and molded goods' waφage, whose deficiency can be improved upon by the combined use of mineral fillers with the glass fibers. Control Example 2, which had too high a melt viscosity, showed a resin filling which was unsatisfactory under the usual injection molding conditions, giving molded articles that had considerably poor surface appearance. On the other hand, Control Example 3, which had too low a resin viscosity, resulted in extensive resin "nose running", thereby making molding difficult.

Example 10 containing only 60% mineral fillers had inferior tensile strength to that of Control Example 5, but had a higher modulus and better moldability, surface properties, and waφage. The amount of waφage of sheets having a ribbed structure, as shown in Figure 1, was smaller with Examples 2 and 9, containing 15 and 45%, 45% and 15%, of mineral fillers and glass fibers, respectively, compared to Control Example 1 containing 60% of glass fibers alone.

With respect to the effect of glass flakes and a combination of glass flakes and glass fibers, Examples 11-16 containing both glass flakes and glass fibers, compared to Control Example 4 containing no glass flakes, exhibited considerably higher flexural moduli and strengths, as well as tensile strengths, while exhibiting similarly good moldability, surface properties, and low waφage. The low waφage behavior of Examples 11-18 is also evident in their low molding shrinkage and a shrink ratio close to 1 (smaller anisotropy in shrinkage) with better results having a higher glass flake content for preventing waφage. Examples 17 and 18, containing 60% and 70%, respectively, of glass flakes alone show essentially no anisotropy in shrinkage, giving a good low waφage molded article which is also provided with a high flexural modulus. Compared to Control Example 1 containing 60% of glass fibers alone, Examples 11-16, particularly Examples 13, 14, and 15, showed less anisotropic molding shrinkage, as well as a similar high flexural modulus, thereby providing a low waφage, high stiffness molded article. Figures 2 and 3 further specifically compare waφage based on

Examples 11, 12, 16, 17, and Control Example 1.

At total weights of glass flakes and glass fibers held constant at 16%, Examples 11, 12, 16, and 17 demonstrate decreasing waφage with an increase in glass flakes content, compared to Control Example 1, which contained 60% of the glass fibers alone. In particular, Example 17, containing 60% of glass flakes alone, gave an essentially waφage-free good molded article, which was also provided with high stiffness.

A comparison of Example 11 with Example 2 shows that the combination of glass flakes and glass fibers gave even lower waφage and higher physical properties, specifically in terms of tensile strength and notched Izod impact strength, compared to combinations of mineral fillers and glass fibers.

Lastly, the relationship between the amount of reinforcing agent compounded and the melt viscosity of the polyamide resin compositions was such that Control Example 4 had good moldability, but Control Examples 2 and 7 having increased levels of reinforcing agents showed too high a viscosity, although Control Example 4 had 38% reinforcing agent and Control Examples 2 and 7 had 60% reinforcing agent and are based on the same nylon 66. This suggests that merely increasing the levels of reinforcing agents degrades the flow and that it is important to adjust the melt viscosity of the polyamide resin composition.

BRIEF DESCRIPTION OF THE FIGURES Figure 1: This is a diagram showing the state of waφage of the molded articles from the polyamide resin compositions of Control Example 1 and Examples 2 and 9.

Figure 2: This is a diagram showing the state of waφage of the molded articles from the polyamide resin compositions of Examples 11, 12, and 16.

Figure 3: This is a diagram showing the state of waφage of the molded articles from the polyamide resin compositions of Example 17 and Control Example 1.

Claims

1. A polyamide composition comprising

(a) 30-50 parts by weight of a polyamide resin and

(b) 70-50 parts by weight of a combination of glass fibers 5 and either glass flakes or mineral filler, said composition having a melt viscosity, when molding, of 50-200 Pascal seconds at a shear rate of 1,216 sec^-1.

2. A polyamide composition comprising

(a) 30-50 parts by weight of a polyamide resin and 10 (b) 70-50 parts by weight of glass flakes, said composition having a melt viscosity, when molding, of 50-200 Pascal seconds at a shear rate of 1,216 sec^-1.

3. A polyamide composition comprising

(a) 40-50 parts by weight of a polyamide resin and 15 (b) 60-50 parts by weight of a mineral filler, said composition having a melt viscosity, when molding, of 50-200 Pascal seconds at a shear rate of 1,216 sec¹.

20

25

30

35