EP0783538A1

EP0783538A1 - Polymeric blends based on polypropylene and polybutylene terephthalate resins

Info

Publication number: EP0783538A1
Application number: EP94930367A
Authority: EP
Inventors: Hans Kristian Kotlar; Guo Hua Hu; Yi Jun Sun; Morand Lambla, Deceased
Original assignee: Borealis AS
Current assignee: Borealis AS
Priority date: 1994-08-26
Filing date: 1994-08-26
Publication date: 1997-07-16
Also published as: WO1996006871A1; BR9408611A

Abstract

A process of producing a compatibilized blend of polypropylene and polybutylene terephthalate resins by a continuous single step extrusion process performed in an extruder, preferably in an inert atmosphere, comprising the steps of: (i) introducing into a first feed zone of the extruder a polypropylene polymer, an allylepoxy compound, a styrenic compound and a peroxide initiator; the reagents being heated to a temperature which is higher than the melting point of the polypropylene polymer and the decomposition temperature of said peroxide while being subjected to continuous intimate mixing; and (ii) introducing a polybutylene terephthalate into a second feed zone of the extruder downstream of the first one, at a position where said polypropylene polymer of step (i) is in a molten state to achieve a melting and intimate mixing of said polybutylene terephthalate with said melted polypropylene resin; and (iii) extruding the obtained blend. A compatibilized blend of polypropylene and polybutylene terephthalate polymers, having synergistic properties of both said polymers, is obtained by the process. The obtained blend may be used as an engineering plastic in manufacturing articles for technical use.

Description

Polymeric blends based on polypropylene and polybutylene terephthalate resins

Technical field.

This invention relates to a process of producing a polymeric blend based on polypropylene (PP) and polybutylene terephthalate (PBT) polymers, and a compatibilized blend of polypropylene and polybutylene terephthalate polymers, having synergistic properties of both said polymers produced by the process. More particular the invention relates to a process comprising a simultaneous grafting of two different monomers, an allylepoxy compound and a styrenic compound, onto the polypropylene polymer resin, and a subsequent blending of the grafted resin with polybutylene terephthalate, the entire process being performed at grafting conditions. This in situ grafting and blending process is accomplished with the polyme¬ ric resins in their molten state by means of a singel step reactive extrusion process. The obtained blends have high elongations at break and improved impact strengths.

Background art

Modifications of the chemically inert polypropylene by grafting in order to achieve compositions having specific properties, is previously known. Such grafting processes may be performed at extrusion conditions. Further, the blending of a grafted polypropylene resin with a different polymer resin to obtain compositions having improved properties, is also known. EP 0.280.454 discloses a method for the grafting of at least two different monomers onto molten hydrocarbon poly¬ mer. In embodiments the polymer is a homopolymer of propylene, a high density polyethylene or a linear low density polyethy¬ lene, and the monomers are styrene or maleic anhydride. The process is operated in the absence of an initiator for the grafting reaction and in the substantial absence of antioxi- dant in the polymer, by the use of an extruder at extrusion conditions.

NO patent application no. 924786 relates to polypro¬ pylene compositions grafted with glycidyl acrylate or glycidyl methacrylate (GMA) using an organic peroxide as a radicalgene- rating agent, performed by kneading the melted mixture, prefe- rably in an inert atmosphere.

US 5.079.295 relates to a thermoplastic composition comprising a polyphenylene ether resin, a modified propylene polymer and a rubbery substance. The polypropylene component is grafted with a styrene-based monomer and an unsaturated carboxylic acid or its derivative. The patent describes a number of grafting processes, however in- the working examples 100 parts by weight of polypropylene are mixed with one part of maleic anhydride (MAH), one part of styrene and one part of peroxide in a mixer, and subsequently extruded at 220°C. In another embodiment an aquous dispersion of polypropylene, styrene and MAH introduced into an autoclave is allowed to react at 120°C. In further embodiments the MAH of the disper¬ sion is replaced by glycidyl methacrylate or glycidyl acry- late.

The use of different grades of polypropylene com¬ positions in manufacturing a broad range of technical articles is generally accepted, but for many end uses polypropylene has a too low stiffness. On the other hand, polybutylene tereph- thalate has a high stiffness, but is difficult to process because of the required high processing temperature, and it is prone to hydrolysis. Besides that, polybutylene terephthalate is a high-priced resin. Therefore, it will be desirable to combine polypropylene and polybutylene terephthalate into one blend having intermediate properties, and being processable in equipment used to process polypropylene.

To achieve a homogeneous blend of polypropylene and polybutylene terephthalate the use of a compatibilizer will normally be expected to be necessary. However, if the poly- propylene could be grafted with a suitable compound, the separate compatibilizer might be omitted. In grafting pro¬ cesses to obtain compositions retaining the desired physical and mechanical properties of the virgin resins chain scissions have to be avoided. When only one type of monomer such as an allylepoxy compound is used in a polypropylene grafting pro¬ cess, the achieved grafting efficiency is poor and accompanied by a heavy degradation of the polypropylene backbone by chain scission in the β-position and simultaneous homopolymerization of the added monomer. This problem may be solved by performing the grafting and the blending processes in the presence of a compound having a stabilizing effect on the intermediate polymeric radicals created during the processes. Moreover, a continuous grafting and blending process combined in one operation would have obvious advantages, both economically and technically.

Summary of the invention

It has now surprisingly been found that a homogeneous blend of polypropylene and polybutylene terephthalate can be achieved when the polypropylene is grafted simultaneously with an allylepoxy compound and a styrenic compound, and then immediately blended with the polybutylene terephthalate by the use of a singel step extrusion process. The present invention thus provides a process of producing an in situ compatibilized blend of polypropylene and polybutylene terephthalate polymers by a continuous single step extrusion process performed in an extruder, preferably in an inert atmosphere, comprising the steps of:

(i) introducing into a first feed zone of the extruder a polypropylene polymer; an allylepoxy compound having the formula:

in which R is H or a C^ alkyl; R_x is -(CH₂)_n-; -C(0)0-(CH₂)_n-; or -(CH₂)_n-0-; and n is an integer of 1 to 4; a styrenic compound having the formula:

in which R₂ is H, OH, CH₃ or allyl; and a peroxide as an initiator, the reagents being heated to a temperature which is higher than the melting point of the polypropylene polymer and the decomposition temperature of said peroxide while being subjec- ted to continuous intimate mixing, and

(ii) introducing a neat polybutylene terephthalate into a second feed zone of the extruder downstream the first one at a position where said polypropylene polymer of step (i) is in a molten state to achieve a melting and intimate mixing of said polybutylene terephthalate with said melted polypro¬ pylene resin, and

(iii) extruding the obtained blend according to methods known per se. The present invention also provides a compatibilized blend of polypropylene and polybutylene terephthalate having synergistic properties of both said polymers produced by the process described above.

The obtained blend may be used as an engineering plastic in the manufacturing of articles for technical use.

Detailed description of the invention

It is an object of the present invention to provide a process of producing a compatibilized blend of a polypropylene resin having an allylepoxy compound and a styrenic compound grafted thereon, with a polybutylene terephthalate resin by an extrusion process, which process being performed at conditions resulting in substantially no degradation of the polypropylene and polybutylene terephthalate resins by chain scissions. The polypropylene resins that may be used in the in situ grafting and blending process comprise polypropylene homopolymers, and copolymers of propylene with ethylene and/or butadiene, in particular resins having a molecular weight of 150,000 to 500,000 and a melt flow index of 0.2 to 100 g/10 minutes, preferably 0.2 to 50 g/10 minutes, determined at

230^βC and 2.16 kg load according to the method of ASTM D 1238. Said copolymers have an ethylene content preferably between 0 and 20 % by weight. The polypropylene resin may be used in the form of granules or as a powder, preferbly as a flowable powder.

Polybutylene terephthalate resins of commercial grades in the form of conventional pellets without any further additives, may be used. It is essential that the polybutylene terephthalate resin has a certain amount of free carboxylic and hydroxylic end groups, preferably at least 0.42 meq/kg of COOH and 0.35 meq/kg of OH groups. However, resins having lower amounts of said groups may also give blends of accep¬ table properties.

The monomeric allylepoxy compound to be grafted onto the polymer chains must contain polar or" functional substi- tuents. Said monomers are preferably chosen from the group comprising allylepoxy compounds having the formula: 0

in which R is H or a C₁._i alkyl; R_j is -(CH₂)_n-; -C(0)0-(CH₂)_n-; 5 or -(CH₂)_n-0-; and n is an integer of 1 to 4. Preferably R is H or CH₃, more preferably CH₃. R^ is preferably -C(0)0-(CH₂)_n-. Thus the most preferred compound is glycidyl methacrylate. Suitable styrenic compounds are those having the formula:

allyl. Preferably R₂ is H, making styrene the preferred styrenic compound. The styrenic compound 5 is assumed to stabilize by delocalization the free radical species being present during the grafting process. Other compounds than those mentioned above having conjugated unsatu- rated double bonds, such as quinone, may give similar results. To initiate the grafting process any free radical o generating peroxide compound known in the art having a suit¬ able decomposition temperature, may be used. In particular the following peroxide initiators have been found to give accep¬ table results: 2,5-bis(tert-butylperoxy)-2,5-dimetylhexane ( "DHBP" ), 2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne 5 ( "Trigonox-145" ) and bis(tert-butylperoxyisopropyl)benzene ( "Perkadox-14" ), of which the latter is most preferably used.

Intimate mixing of the polypropylene resin, the initiator and the grafting monomers at a temperature higher than both the melting point of the polypropylene resin and the peroxide decomposition temperature is a prerequisite for achieveing a desired degree of grafting onto the polypropylene resin.

The process according to the present invention is

5 most suitably performed by the use of an extruder, preferably a double screw extruder having corotating intermeshing screws of sufficient length, for example L/D = 42. The extruder must be provided with two feed hoppers, one located at the main feeding point of the extruder, and the other one located in a ιo distance of approximately 0.4 L downstream of the first one. Each hopper is connected with a feeding device, from which the starting material is continuously fed at a controlled rate. The extruder barrel thus becomes divided into two main zones, a first zone between the two hoppers, and a second zone be- i5 tween the second hopper and the die.

The polypropylene resin is fed to the first feeder together with the allylepoxy compound, the styrenic compound and the initiator, which may be premixed before being conveyed to the first feed hopper. In a specific embodyment of the

2o invention the grafting monomers used are glycidylalkyl acry¬ late and styrene. It is convenient first to mix the glycidyl¬ alkyl acrylate with styrene and then dissolve the peroxide initiator in this mixture, which subsequently is mixed into the polypropylene powder. The polypropylene melt mixture in

25 the first main extruder zone should comprise a concentration of glycidyl methacrylate of 1 to 10% by weight, preferably 2 to 5% by weight, and a concentration of styrene of 1 to 10% by weight, preferably 2 to 5% by weight, calculated on the basis of the neat polypropylene resin. An appropriate amount of

30 initiator is approximately 0.2 to 0.3 % by weight of the polypropylene resin.

The polybutylene terephthalate is conveyed to the second hopper, from which it is introduced into the extruder. The weight ratio of the polypropylene resin to the

35 polybutylene terephthalate resin fed to the extruder is chosen in respect of the desired properties of the final material. To obtain an intimate mixing of the polypropylene resin and the grafting monomers, as well as a reasonable decomposition rate of the free radical generating initiator, the temperature in the first main extruder zone should be approximately 200°C. At this temperture the preferred peroxide initiator "Perkadox-14" has a half-life of 6.5 seconds. In the second main extruder zone the temperature should be raised to s about 240°C to ensure that the introduced polybutylene tereph¬ thalate is rapidly melted and mixed with the molten grafted polypropylene.

One problem encountered in the grafting and blending process is a possible contamination of the reagents with o oxygen from the surrounding air. Oxygen will react spontan- ously with the generated radicals, giving oxy-radicals which are particularly active agents in polymer chain scissions. Therefore, the present extrusion process is preferably carried out in an inert atmosphere, most preferably in a nitrogen s atmosphere. This can readily be accomplished by flushing the feed hoppers with nitrogen.

The polymer chain scissions and hence the molecular weights of the present blends can be controlled by the use of the comonomer system described herein. In the grafting reac- 0 tions taking place the styrenic compound will function as a chain transfer agent. Compared to traditional grafting and blending processes without the use of a chain transfer agent, the present process can be performed with less degradation of the polypropylene resin. 5 The styrenic compound also acts as a comonomer, reacting with the allylepoxy monomer to give random copoly¬ mers. Thus, the side chains grafted onto the polypropylene backbone are random copolymers of said two species. The simul¬ taneous grafting with the two monomers provides a synergistic effect resulting in a higher grafting efficiency and a higher amount of monomers being grafted onto the polypropylene resin compared to other grafting processes, which increases its number of polar groups. It is assumed that said polar groups will react with the functional end groups of the polybutylene terephthalate at the interfaces between the polypropylene and polybutylene terephthalate fractions. Hence, there will be a minor amount of crosslinking between the polypropylene and polybutylene terephthalate resins with the allylepoxy compound as the bridging species. The polymeric blends of the present invention can be used as a single resin without any further compounding. Con¬ ventional additives such as colorants, antistatics, UV stabi¬ lizers, etc. may be added if desired, in manners well known in the art.

The present blends of polypropylene and polybutylene terephthalate show synergistic effects in respect of such properties as stiffnes, impact strength and elongation at yield, which are clearly demonstrated by the values of said properties shown in Table 3 below. The obtained blends behave like one material in use, and the obtained properties being between the properties of the neat resin components, is a strong indication of the blend being truly homogeneous.

The blends of the present invention can advanta- geously be used as an engineering plastic. More particular it can be used in appliances, automotives and in articles for technical use. The blends can be processed by all ordinary plastic processing techniques used for polypropylene resins, such as extrusion and injection moulding. The invention will now be illustrated and described in more detail by way of examples, which have not to be con¬ strued as limiting the invention.

Examples The extruder used in the examples is a ZSK30 Werner &

Pfleiderer double screw extruder with co-rotating intermeshing screws of length = 1230 mm and L/D = 42, operated at a rate of 150 rpm. The extruder is provided with two feed hoppers, a first hopper through which the polypropylene resin is introdu- ced into the extruder at the starting end of the barrel, and at a distance of 1^ = 0.378L downstream of the first hopper, a second hopper through which the polybutylene terephthalate resin is introduced into the extruder, i.e a first feeding point and a second feeding point, respectively. The temperatu- res of the extruder are adjusted at 200^βC in the first zone between said two feed hoppers, and at 240°C downstream of the second feed hopper and at the die.

The obtained compositions are subjected to mechanical testing according to standardized procedures. Based on the stress-strain curves obtained by tensile testing modulus of elasticity, stress at yield and elongation at break are deter¬ mined. The modulus of elasticity is determined according to ISO 527, while the stress at yield and elongation at break are s determined according to ASTM D 638. The impact strength is determined as the total energy of break by dropping a load onto a 3 mm thick sheet of 0^βC according- to standardized ISO procedure of ISO.

Obtained values of mechanical properties are listed o in Table 3, where corresponding figures for the neat polypro¬ pylene and polybutylene resins used in the examples are pre¬ sented for reasons of comparison. In Table 3 the concentration of glycidyl methacrylate (GMA) in the starting reaction mix¬ ture is denoted [GMA]^ 5

Examples 1 - 3

Polypropylene powder moistened with a liquid mixture of glycidyl methacrylate (GMA), styrene and a peroxide initia¬ tor ( "Perkadox-14" ) is introduced into the extruder barrel at 0 the first feeding point and extruded at the conditions speci¬ fied above. Polybutylene terephthalate pellets with no mono¬ mers or initiator added is introduced into the extruder barrel at the second feeding point. The PP/GMA/styrene/initiator weight ratios are 100/3.0/3.0/0.3, giving a styrene/GMA ratio 5 of 1.4 on a molar basis. The experimental details are given in table 1. The physical properties of the obtained blends are presented in table 3.

0

5 Table 1

1. feeding 2. feeding

Example point point Proportions

PP/GMA/styrene/ PBT pellets of PP/PBT peroxid kg/h kg/h % by weight

1 3.5 1.5 70/30

2 2.0 2.0 50/50

3 1.2 2.8 30/70

Examples 4 and 5

The procedure and extrusion conditions are as specified in example 1, except that the PP/GMA/styrene/initiator weight ratios are different. In example 4 said weight ratios are 100/1.4/1.4/0.3, and in example 5 they are 100/1.0/1.0/0.20. In both examples 4 and 5 there is a constant styrene/GMA ratio of 1.4 on a molar basis. The physical properties of the obtained blends are presented in table 3.

Comparative example 6 The procedure and extrusion conditions of example 5 are used, except that the feeding rate of the premixed polypropylene resin is 2.8 kg/h, and the feeding rate of the neat polybuty¬ lene terephthalate pellets is 1.2 kg/h, and both the polypro¬ pylene premix and the polybutylene terephthalate are fed to the first feed hopper and together continuously introduced into the extruder barrel without being exposed to any parti¬ cular mixing. Experimental details are given in table 2. The physical properties of the obtained blend are presented in table 3.

Comparative examples 7 and 8

Polypropylene powder without any monomers or peroxide initia¬ tor added, and neat polybutylene terephthalate pellets are separately introduced into the extruder barrel at the first and second feeding points, respectively, and extruded at the conditions specified above. The feeding rates of the polypro¬ pylene powder and the polybutylene terephthalate pellets are as indicated in table 2. The mechanical properties of the obtained blends are presented in table 3.

Table 2

1. feeding 2. feeding

Example point point Proportions

PP powder PBT pellets of PP/PBT kg/h kg/h % by weight

Comp. 6 3.5 1.5 70/30

Co p. 7 2.0 2.0 50/50

Comp. 8 1.2 2.8 30/70

Claims

PATENT CLAIMS

1. A process of producing an in situ compatibilized blend of polypropylene and polybutylene terephtalate polymers by a continuous single step extrusion process performed in an extruder, preferably in an inert atmosphere, c h a r a c t e r i z e d i n comprising the steps of:

(i) introducing into a first feed zone of the extruder a polypropylene polymer; an allylepoxy compound having the formula

in which R₂ is H, OH, CH₃ or allyl; and a peroxide as an initiator, the reagents being heated to a temperature which is higher than the melting point of the polypropylene polymer and the decomposition temperature of said peroxide while being subjec¬ ted to continuous intimate mixing, and

(ii) introducing a polybutylene terephthalate into a second feed zone of the extruder downstream the first one, at _a position where said polypropylene polymer of step (i) is in a molten state to achieve a melting and intimate mixing of said polybutylene terephthalate with said melted polypropylene resin, and

(iii) extruding the obtained blend according to methods known per se.

2. The process of claim 1, c h a r a c t e r i z e d i n that the polypropylene is fed to the extruder in the form of a powder premixed with the allylepoxy compound, the styrenic compound and the peroxide.

3. The process of claim 1,

5 c h a r a c t e r i z e d i n that the allylepoxy compound is glycidyl methacrylate.

4. The process of claim 3, c h a r a c t e r i z e d i n that the concentration of o glycidylmethacrylate in the polypropylene melt mixture in the first extruder zone is in the range of 1 to 10% by weight, calculated on the polypropylene polymer.

5. The process of claim 1, s c h a r a c t e r i z e d i n that the styrenic compound is - styrene.

6. The process of claim 5, c h a r a c t e r i z e d i n that the concentration of o styrene in the polypropylene melt mixture in the first extru¬ der zone is in the range of 1 to 10% by weight, calculated on the polypropylene polymer.

7. The process of claim 1, 5 c h a r a c t e r i z e d i n that the peroxide initiator is bis(tert-butylperoxyisopropyl)benzene and that the concen¬ tration of said peroxide in the polypropylene melt mixture in the first extruder zone is in the range of 0.2 to 0.3 % by weight, calculated on the polypropylene polymer. 0

8. The process of claim 1, c h a r a c t e r i z e d i n that the polybutylene tereph¬ thalate resin contains at least 0.42 meq/kg of free carboxylic end groups and at least 0.35 meq/kg of free hydroxylic end 5 groups.

9. A compatibilized blend of polypropylene and polybuty¬ lene terephthalate polymers, having synergistic properties of both said polymers, c h a r a c t e r i z e d i n being obtained by the process according to claims 1 to 8.

10. A use of the compatibilized blend according to claims s 1 to 9, as an engineering plastic in the manufacturing of articles for technical use.

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