GB2286595A

GB2286595A - Method of preparing cast polyamide having increased stiffness

Info

Publication number: GB2286595A
Application number: GB9502549A
Authority: GB
Inventors: Ernst Grigat; Aziz El-Sayed; Edgar Ostlinning; Karste-Josef Idel
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1994-02-10
Filing date: 1995-02-09
Publication date: 1995-08-23
Also published as: ITRM950066A1; DE4404221A1; GB9502549D0; NL9500242A; JPH07238163A; ITRM950066A0

Description

2286595 Method of preparing cast polyamide having increased stiffness This

invention relates to increasing the stiffness of cast polyamide reinforced with glass fibres, and optionally with kaolin or chalk in addition, by the addition of polystyrene to the lactam melt and subsequent polymerization.

Cast polyamide is a polyamide prepared by batch anionic polymerization and is used for the production of mouldings, for example cable deflection rollers or parts used in the construction of heavy machinery, which are difficult to produce by injection moulding, glass forming and other common thermoplastic processing methods. An increase in stiffness is advantageous for many of these applications.

The range of auxiliary processing agents which can be used in cast polyamide is severely restricted by the sensitivity of the anionic polymerization to interfering substances, particularly protic compounds, and due to the production process, in which the fluid melt first has to be capable of being pumped and then has to be capable of standing without loss of homogeneity for the polymerization process.

The impact strength of cast polyamide can be improved by introducing additives (with a corresponding loss of stiffness, see DE-A 425 318, for example), but no additive incorporation procedure for increasing the stiffness which goes beyond a pure filling or reinforcement procedure has hitherto been known.

The addition of polystyrene to the cast polyamide melt and subsequent polymerization are known (DE-A 4 100 314). However, an additional stiffening effect is neither described nor even suggested, and nor does this teaching include the particularly advantageous combination with a system which is homogeneously reinforced with glass fibres and optionally by kaolin or chalk in addition.

One route for increasing the stiffness is the introduction of glass fibre mats into the mould, followed by casting cast polyamide melt over them. However this route is an involved one, and is therefore very expensive. Another route is the introduction of milled glass fibres or other fillers into the melt (see DE-A 3 936 022, 4 124 319 as representative examples). Additional stiffness of the glass fibre-reinforced cast polyamide due to the polymers used is not observed and is not even suggested.

is It has surprisingly been found that the addition of polymers with a high content of styrene, particularly polystyrene itself, leads to an increase in the stiffness of cast polyamides reinforced with glass fibres and optionally with kaolin or chalk in addition, the stiffness being defined as the modulus of elasticity in bending, measured according to DIN 53 452. It is particularly gratifying and useful that the combination of polystyrene with fillers or reinforcing materials leads to a further improvement in the stiffness of the already reinforced cast polyamide. This effect is found both in the freshly moulded state and in the conditioned state, the order of magnitude of this effect indicating that the improvement of the stiffness in the conditioned state is not due to a reduction in water absorption, but must have other causes.

The improvement in stiffness is also surprising because polystyrene has a stiffness comparable with that of polyamide. No increase in modulus of the modified cast polyamide would be expected, using the calculated mean elastic modulus based on the polyamide fraction and the polystyrene fraction to predict this.

The present invention relates to a method of preparing cast polyamide having increased stiffness, characterized in that two mixtures A and B are first prepared in a first stage:

A) the following are homogenized in a lactam melt at 800C to 1600C, preferably 1OWC to OWC:

1 weight % to 30 weight %, preferably 2 weight % to 15 weight %, most preferably 3 weight % to 8 weight % (based on the lactam), of a polymer comprising at least 91 weight % styrene, preferably polystyrene; 5 to 25 weight preferably 12 to 18 weight % (based on the lactam), of glass fibres, preferably milled glass ("milled fibers"); 0 to 15 weight %, preferably 0 to 10 weight % (based on the lactam), of chalk or preferably kaolin; Ar and 0.15 weight % to 1 weight %, 0.25 weight % to 0.5 weight %, of catalyst (calculated as undiluted catalyst, for example pure lactam), preferably sodium lactamate, optionally dissolved in up to ten times the amount of caprolactam (based on the catalyst); 0 to 25 weight %, preferably 0 to 15 weight %, most preferably 0 to 8 weight % (based on the lactam), of additives, such as thixotropic agents, impact strength modifiers, colorants, demoulding agents or stabilizers, for example B) the following are homogenized in a lactam melt at 800C to 1600C, preferably 1OWC to OWC: 1 weight % to 30 weight %, preferably 2 weight % to 15 weight %, most preferably 3 weight % to 8 weight % (based on the lactam), of a polymer comprising at least 91 weight % styrene, preferably polystyrene; 5 to 25 weight %, preferably 12 to 18 weight % (based on the lactam), of glass fibres, preferably milled glass ("milled fibers"); 0 to 15 weight %, preferably 0 to 10 weight % (based on the lactam), of chalk or kaolin; and 0.5 weight % to 3 weight %, preferably 0.7 weight % to 1.5 weight %, of activator 0 to 15 weight %, preferably 0 to 8 weight % (based on the lactam), of additives, such as thixotropic agents, impact strength modifiers, colorants, demoulding agents or stabilizers, for example and these mixtures are then subjected to polymerization in a second stage at a temperature between 1OWC and 16WC, preferably between HO'C and OWC, by mixing them together in the same proportion (40 weight % A and 60 weight % B to 60 weight % A and 40 weight % B).

There is also another embodiment of the method according to the invention, wherein the overall composition of the polymerization mixture A plus B is obtained by partitioning into partial mixtures other than in the aforementioned procedure. Either the activator or the catalyst only may be present in each separate partial mixture. In this connection it must be ensured that no other components which are incompatible with each other are added to the same partial mixture (for example certain impact strength modifiers with certain activators). Admixture is effected as in the embodiment described previously, at a temperature between 1 OWC and 16WC, preferably between 11 O'C and 130 ', C, by mixing together in quantitative proportions such that a homogeneous mixture of A plus B is obtained.

All unsubstituted lactams with a ring size: 6 may be used as the lactam. Laurolactam is preferred, and E-caprolactam is particularly preferred.

Copolymers of styrene with ethylene, propylene or substituted styrenes, for example, may be used as the polymer comprising at least 91 weight % styrene, preferably 0 polystyrene, wherein in exceptional cases the increase in stiffness according to the invention can be obtained even with styrene contents less than 91 weight %. Polystyrene is preferred; polystyrene with a molecular weight higher than average is particularly preferred.

A further, preferred variant of the method consists of enriching the styrene-rich polymer, preferably polystyrene, before dissolution with lactam, with an amount of 1 weight % to 200 weight %, preferably 20 weight % to 130 weight %, most preferably 50 weight 0 % to 100 weight %, of lactam (based on the polymer). This can advantageously be effected in an extruder or kneader. The advantage is a significant reduction of the time of dissolution in the lactam melt.

The catalyst according to the concept of the invention is an alkali (Na, K) or alkaline earth (Mg, Ca) lactamate, preferably as a solution in lactam. Sodium caprolactamate in E-caprolactam is particularly preferred.

The activators according to the concept of the invention may be N-acyl lactams or acid 0 chlorides, or preferably aliphatic isocyanates, oligomers of hexamethylene diisocyanate C> being particularly preferred. Both the pure substance and also, preferably, a solution, in N-methyl pyrrolidone for example, may serve as the activator.

9 k Other additives may for example comprise impact strength modifiers, flame retardants, nucleating agents, colorants, filler oils, stabilizers, surface enhancers and demoulding agents.

The casting polymerization apparatus should preferably have a melt circuit for each separate melt as far as the mixing head, through which circuit the melt is kept in motion in the flexible tubes and lines by continuous pumping. However, such a circuit is not absolutely necessary for carrying out the method according to the invention.

The present invention also relates to all mouldings or articles produced by the method according to the invention.

The present invention likewise relates to mouldings or articles produced from stiffer polyamides by the addition of the styrene-rich polymer, preferably polystyrene.

EXAMPLES

An 18 1 % solution of sodium lactamate in c-caprolactam was employed as the catalyst (Catalyst NI, manufactured by Bayer AG). The activator was an oligomer of hexamethyl diisocyanate dissolved in N-methyl pyrrolidone (Activator 8107 manufactured by Bayer AG). The thixotropic agent was a mixture of a silicate with highly-dispersed alumina (about 4.5: 1 to 6.2: 1) with a DIN ISO 787/XI tamped density of 50 g/1 and a BET specific surface of 100 to 250 rn?1g. An Ultraturrax T 25 manufactured by Jahnke & Kunkel, Ika Labortechnik, Staufen i. Br., was used for thixotropic mixing. Stirring was effected for 1 minute at a speed of 20, 000 to 21,000 rpm and for 1 minute at maximum speed (about 24,000 rpm). The polystyrene used was polystyrene 168N manufactured by BASF AG.

Comparative Example 1 A: A thixotropic mixture of 237 g E-caprolactam, 35 g catalyst and 8 g thixotropic agent was produced at 12TC; 52.5 g glass (milled fiber) and 35 g kaolin were then stirred in.

B: A thixotropic mixture of 264 g c-caprolactam, 8 g activator and 8 g thixotropic agent was produced at 120'C; 52.5 g glass (milled fiber) and 35 g kaolin were then stirred in.

The melts were mixed and polymerized for a maximum of four minutes.

The blocks were granulated and injection-moulded in the form of bars of edge lengths x 10 x 4 mm. The modulus of elasticity in bending obtained according to DIN 53 452 was 5254 N/ram? (dry) and 2054 N/MM2 (conditioned according to ISO 1110, water absorption 2.26 Z t, Example 2

A:

10.5 g polystyrene were dissolved in 203 g cs-caprolactam at 120'C. 42 g catalyst and 7 g thixotropic agent were then added and a thixotropic mixture was produced. 52.5 g glass (milled fiber) and 35 g kaolin were then stirred in.

B: 10.5 g polystyrene were dissolved in 237 g E-caprolactam at 120'C. 9 g activator and 7 g thixotropic agent were then added and a thixotropic mixture was produced. 52.5 g glass (milled fiber) and 35 g kaolin were then stirred in.

The melts were mixed and polymerised for a maximum of three minutes.

The blocks were granulated and injection-moulded in the form of bars of edge lengths 80 x 10 x 4 mm. The modulus of elasticity in bending obtained according to DIN 53 452 was 5459 Wrnm? (dry) and 2313 N/mM2 (conditioned according to ISO 1110, water absorption 2.23 %).

Example 3

17,5 g polystyrene were dissolved in 197 g c-caprolactam at 1200C. 42 g catalyst and 6 g thixotropic agent were then added and a thixotropic mixture was produced. 52.5 g glass (milled fiber) and 35 g kaolin were then stirred in.

B: 17.5 g polystyrene were dissolved in 230 g E-caprolactam at 120'C. 9 g activator and 6 g thixotropic agent were then added and a thixotropic mixture was produced. 52.5 g glass (milled fiber) and 35 g kaolin were then stirred in.

The melts were mixed and polymerised for a maximum of three minutes.

The blocks were granulated and injection-moulded in the form of bars of edge lengths c x 10 x 4 mm. The modulus of elasticity in bending obtained according to DIN 53 452 was 5591 N/mrr (dry) and 2398 N1min' (conditioned according to ISO 1110, water absorption 2.20 %).

it, t

Claims

CLAIMS 1. A method of preparing cast polyamide having increased stiffness, characterized in that two mixtures A and B are first prepared in a first stage: A) the following are homogenized in a lactam melt at 800C to 1600C, preferably 1000C to 130'C: 1 weight % to 30 weight % (based on the lactam), of a polymer comprising at least 91 weight % styrene, 5 to 25 weight % (based on the lactam), of glass fibres, 0 to 15 weight % (based on the lactam), of chalk and/or preferably kaolin; and 0. 15 weight % to 1 weight % of catalyst, optionally dissolved in up to ten times the amount of caprolactam (based on the catalyst); 0 to 25 weight % (based on the lactam), of additives, such as thixotropic agents, impact strength modifiers, colorants, demoulding agents or stabilizers, for example B) the following are homogenized in a lactam melt at 800C to 160OC:

1 weight % to 30 weight % (based on the lactam), of a polymer comprising at least 91 weight % styrene, (based on the lactam), 5 weight % to 25 weight % of glass fibres, 0 to 15 weight % (based on the lactam), of chalk and/or kaolin; and 0.5 weight % to 3 weight % of activator 0 to 15 weight % (based on the lactam), of additives, such as thixotropic agents, impact strength modifiers, colorants, demoulding agents or stabilizers, for example and these mixtures are then subjected to polymerization in a second stage at a temperature between 1 OTC and 160'C, preferably between 11 O'C and OTC, by mixing them together in the same proportion.

A method according to claim 1, characterized in that the overall composition of the polymerization mixture A plus B is obtained by partitioning into partial mixtures other than in claim 1, wherein either activator or catalyst only may be present in each individual partial mixture.

3. A method according to claim 1 or claim 2, characterized in that the polymer is premixed from at least 91 weight % styrene with 1 weight % to 200 weight % lactam, based on the polymer.

4. A method according to claims 1 to 3, characterized in that the lactam is laurolactam or E-caprolactam.

5. A method according to claims 1 to 4, characterized in that the glass fibres are milled (milled fibers).

6.

7.

8.

Mouldings obtained from the materials according to claim 1 A method for manufacturing a cast polyamide substantially as hereinbefore described.

A cast polyamide prepared according to any of claims 1 to 7.

t t