WO1990003411A1

WO1990003411A1 - Method of preparing reinforced rubber and reinforced rubber products

Info

Publication number: WO1990003411A1
Application number: PCT/SE1989/000511
Authority: WO
Inventors: Jan Erik Persson; Bengt Rånby
Original assignee: Institutet Polymerutveckling Ab
Priority date: 1988-09-22
Filing date: 1989-09-21
Publication date: 1990-04-05
Also published as: SE8803353D0; SE462044B

Abstract

This invention relates to a method of reinforcing rubber with cellulose fibers which have been modified by graft copolymerization, and a fiber reinforced rubber product. Discontinuous cellulose fibers modified by graft copolymerisation are admixed into a conventional vulcanizing rubber mixture of a natural rubber or a synthetical rubber before vulcanization. Said cellulose fibers have been modified by graft copolymerization with at least a monomer of alkyl or alkenyl acrylate, alkyl or alkenyl methacrylate or a copolymer of two or more of said monomers. Compared to untreated cellulose fibers the modified cellulose fibers have a higher adhesion to the rubber matrix. The fiber reinforced rubber product has high strength and modulus.

Description

Method of preparing reinforced rubber and reinforced rubber products

This invention relates to a method of reinforcing rubber with cellulose fibers and a cellulose fiber reinforced rubber product. The cellulose fibers used are modified by graft copolymerization with at least a monomer of alkyl or alkenyl acrylate or alkyl or alkenyl methacrylate or a copolymer of two or more of said monomers In recent years short fiber rein orcement of rubber has become a field of increasing technical and economical interest. Short fiber reinforcement makes possible among others, a simple method of preparing reinforced rubber products. In this respect the cellulose fiber has turned out to be a suitable fiber due to its abundance, good aspect ratio (the ratio between the length and diameter of the fiber) and flexibility; see for example Campbell J.M. Progress of Rubber Technology, Vol. 41, Plastics and Rubber Institute, London (1978) p. 43 - 60.

In earlier attempts to produce cellulose fiber reinforced rubber, the reinforced rubber has shown un¬ satisfactory strength and different attempts have therefore been made to improve the strength of the reinforced rubber. Discontinuous cellulose fibers have been admixed to rubber together with different coupling or bonding agents to give the reinforced rubber properties comparable with the properties of continuous fibers, see for example US-A- 3697364. The effect of vulcanization on the adhesion properties between untreated cellulose fibers and the rubber matrix has recently been studied by Flink P. et al, 3 . Appl. Pol. Sci. 35 (1988), p. 2155 - 2164.

The novel method according to the invention for the reinforcement of rubber uses cellulose fibers modified by graft copolymerization with alkyl or alkenyl acrylates, alkyl or alkenyl methacrylates or copolymers thereof. Surprisingly it has been found that the modified cellulose fibers give the reinforced rubber a strength which is higher than that of rubber products reinforced by un¬ treated cellulose fibers. It is an obvious fact that grafting modifies the surface of the fiber giving better adhesion between the fiber and the rubber matrix. The present method of preparing fiber reinforced rubber is disclosed in claims 1 - 4 and such a reinforced rubber product is defined in claims 5 - 10.

Natural rubber and synethetic rubber can both be used as reinforcement according to the present method. Synthetic rubber types are preferably non polar rubber, such as styrene-butadiene rubber (SBR) butadiene rubber (BR) isoprene rubber (IR), ethylene-propylene-diene rubber (EPDM), ethylene-propene rubber (EPM) or norbornene rubber (PNR). Also chloroprene rubber (CR), acrylic rubber (ACM) or πitrile-butadiene rubber (NBR) can be a possible choice. In this case, reinforced natural rubber, styrene-butadiene rubber and nitrile rubber are of special interest.

Cellulose fibers of different origin with for the application appropriate aspect ratio, i.e. an aspect ratio giving the rubber of choice optimal reinforcement with respect to use, are modified by graft copolymerization with at least a monomer of alkyl or alkenyl acrylate or^* alkyl or alkenyl methacrylate or a copolymer of two or more of said monomers. Suitable monomers are C-j - C-_]2 alkyl acrylate, C-| - C-j£ alkenyl acrylate, C-j - 12 alkyl methacrylate or C-] - C-J2 alkenyl methacrylate and especially suitable are monomers containing alkyl and alkenyl groups, respectively, containing 1 - 8 carbon atoms. Cellulose fibers grafted with butyl acrylate appears to be especially suitable for the reinforcement of rubber with fibers but all grafted cellulose fibers show increased adhesion and give a rubber product with improved strength and low elongation.

Graft copolymerization of said cellulose fibers can be carried out by different grafting methods and the present invention is not delimited to the grafting method exemplified below. Especially the amounts of monomer and cellulose fibers have to be examined in respect of the reactor used and the desired modification of the fibers. The grafting can also be initiated by other methods, such as radiation.

Nevertheless it can be conducted that the grafting method exemplified below, comprising at least a monomer and at least an initiator in the form of a manganese (III) complex, is suitable especially for modification of cellulose fibers at a concentration of initiator of 0.5 to 3 mM, calculated on the Mn^⁺ complex, preferably 1 - 2 mM. The grafting reaction is fast, giving a high yield and the grafting occurs on the surface of the fiber at a monomer concentration in the range of 25 to 200 ml monomer per 200 g cellulose, preferably 50 to 100 ml monomer/ 200 g cellulose. Owing to the monomers being grafted onto the surface of the cellulose fibers, the surface of the fibers is modified and there is increased adhesion between fibers and rubber matrix. This implies that even at a low fiber reinforcement of rubber (about 10 - 40 ?ό) the result¬ ing products will have good stress properties and high modulus. Example 1 Grafting of cellulose fibers

Different monomer mixtures were used for preparing grafted cellulose fibers. In the three cases exemplified below the monomer mixtures comprise, respectively:

I) 50 ml butyl acrylate (BA),

II) 25 ml butyl acrylate and 30 ml allyl acrylate (ABA),

III) 25 ml butyl acrylate and 25 ml crotyl acrylate (CBA). The initiator was prepared for each reaction from three solutions:

Solution A: 14.46 g a P2<-7 x 10 H2O dissolved in distilled water to a volume of 250 ml.

The pH was adjusted to 6 by addition of concentrated sulphuric acid. At this pH the pyrophosphate is in the form of

ions Solution B: . 6.41 g MnSU dissolved in distilled water to a volume of 100 ml Solution C: 1.26 g KMnO^ dissolved in distilled water to a volume of 100 ml For 10 1 reaction volume the following amounts of these solutions were used; 150 ml of solution A to give a H2P2^U7 concentration of 2 m in final solution and 25 ml each of solution B and solution C to give a n^⁺ concentration of 1 mM in final solution. The solutions A and B have to be' mixed before the addition of C to prevent precipilarriόn of nθ2-

In a 10 1 flat flange flask made of glass, equipped with a stirrer, a thermometer, a dropping funnel, a gas inlet tube and a condenser and placed in a ther- mostatted heating mantle 200 g (dry) of cellulose sold under the trade mark Stora Fluff (a bleached sulphate pulp based on pine, fabricated by Stora AB, Sweden and defibrated by MoDo Converting Machinery AB , Sweden) was added. The reactor was closed, and it was deaerated by alternately evacuating and purging with nitrogen. Distilled water, acidified with 11 ml concentrated sulphuric acid and one of the monomers mixtures were added to the reactor to give a total reaction volume of 10 litres. Before addition, water and monomer were deaerated in the same way as the cellulose. Stirring, heating and nitrogen bubbling was started, and when the temperature was 30°C the deaerated initiator was added.

After three hours, the reaction was stopped and the content of the reactor was transferred to a filter funnel and washed with distilled water and methanol. The product was dried to constant weight in vacuum at room temperature.

The results of the grafting reaction, which are given in table 1 , show that the highest yield of polymer on modified cellulose fibers is achieved with the butyl acrylate monomer. Table 1 : Conversion of cellulose fibers by grafting Grafting Add on ' Double bonds in % jmol double bonds in % per g grafted fiber grafted mers

butyl acrylate 17.2 alkyl butyl acrylate 16.8 85 7 crotyl butyl acrylate 15.0 201 15 *)

Add on _ 9 grafted polymer _{χ 10Q} g grafted product

The amounts of remaining double bonds were determined by bromination with pyridine hydrobromide per- bromide at room temperature in a 75:25 % (v/v) mixture of methanol and carbon tetrachloride (See Tunncliffe M.E., 3 . Appl. Pol. Sci, 14, p. 827 (1970).

The grafted cellulose fibers were admixed into conventional rubber mixtures in an amount of 5 to 70 Phr (parts per hundred rubber), preferably 10 - 40 Phr. The rubber samples comprise conventional and for the vulcanization system necessary chemicals, such as an activator and other usual additives and fillers, and a process oil and an antioxidant. The rubber samples were mixed with the grafted cellulose fibers and were then vulcanised by the method described in more detail in example 2 below. Example 2 Production of fiber reinforced rubber

Rubber mixtures of the following formulations were prepared: Component Amount (Phr)

Natural rubber (SMR CV 50) 100

Dutrex 729 (processoil) 4

Stearic acid 2

ZnO 5

Carbon black (N330) 20

TMQ (see below) 2

CBS (see below) 0.5

Sulphur 2.5

Cellulose fibers grafted alt. 10,20,30 or 40 according to Example 1

Remark: Stearic acid, ZnO, CBS and sulphur are vulcaniza¬ tion chemicals, wherein CBS (cyclohexylbenzotriazylsulphen- amide) is an activator, TMQ (polymertrimethyldihydro- chinoline is an antioxidant.

The different rubber samples were mixed in a roll mill to 2 mm thick plates. These plates were then vulcanized at 160°C to t g, the time at which 90 % of the maximum torque is achieved, determined from rheometer curves obtained using a Monsanto oscillating disc rheometer.

The vulcanized plates were examined with an Instron 1122 Universal Testing Instrument. The stress strain curves were recorded at drawing with 50 mm/min. The test speciments were of dumbbell shape according to ASTM Die C and cut parallel to the fiber orientation. The results are shown in Figures 1 and 2, where Figure 1 shows stress strain curves of natural rubber reinforced with different modified cellulose fibers and untreated fibers respectively.

Figure 2 shows Young's modulus of reinforced rubber as a function of admixed cellulose fibers. The following notations are used: C = untreated cellulose fiber, ABA = cellulose fibers grafted with allyl butyl acrylate, BA = cellulose fibers grafted with butyl acrylate and CBA = cellulose fibers grafted with crotyl butyyl acrylate. In Figure 1 all curves are drawn for a fiber concentration of 30 Phr. All the curves for modified cellulose fibers have the same general shape. First a rather sharp rise, a small decrease followed by a plateau region, and then a second rise before break. Compared to untreated fibers all modified fibers give reinforced rubber products with greater strength. The best reinforcement effect was achieved with the butyl acrylate grafted fibers.

The modulus of an unfilled rubber sample is proportional to its crosslinking density. For a series of fiber filled rubbers, the modulus can be regarded as a relative measure of the adhesion between the matrix and the fiber. In Figure 2 the modulus as a function of fiber concentration is shown. The results show that the adhesion between fiber and rubber is best for the butyl acrylated grafted fibers. The adhesion between rubber matrix and fibers grafted with allyl butyl acrylate and crotyl butyl acrylate. respectively being slightly lower indicates that the linear polybutyl acrylate chains are more flexible and thus better penetrate the matrix. Allyl acrylate and crotyl acrylate are both crosslinked partly during graft copolymerization, probably affecting the adhesion between the fiber and the rubber matrix. If this crosslinking can be restrained, the adhesion of allyl and crotyl acrylate grafted fibers should increase.

Claims

CL A I MS

1. A method of reinforcing rubber with fibers by admixing discontinuous cellulose fibers into a vulcaniz¬ ing rubber mixture of natural rubber or synthetical rubber before vulcanization, characterized in that the admixed cellulose fibers have been modified by graft copolymeriza¬ tion with at least a monomer of alkyl or alkenyl acrylate, alkyl or alkenyl methacrylate or a copolymer of two or more of said monomers.

2. A method ' according to claim 1, characterized in that the admixed cellulose fibers have been modified by graft copolymerization with at least a C-i - C-J2 alkyl acrylate, C₁ - C₁₂ alkenyl acrylate, C-_j - C₁₂ alkyl- ethacrylate or C-_j - C-_j2 alkenyl methacrylate, or a copolymer of two or more of said monomers and with an initiator in the from of a Mn^⁺ complex.

3. A method according to claim 1 or claim 2, characterized in that the cellulose fibers have been modified by graft copolymerization with butyl acrylate.

4. A method according to any of the claims 1 - 3, characterized in that modified cellulose fibers are admixed into the rubber mixture in an amount of 5 - 70 Phr (parts by weight per hundred parts of rubber), preferably 10 - 40 Phr.

5. A rubber product reinforced with discontinuous cellulose fibers, characterized in that cellulose fibers are admixed into a vulcanizing rubber mixture of natural rubber or synthetical rubber before the vulcanization, wherein the cellulose fibers have been modified by graft copolymerization with at least a monomer of alkyl or alkenyl acrylate, alkyl or alkenyl methacrylate or a copolymer of two or more of said monomers.

6. A rubber product according to claim 5, characterized in that the rubber mixture comprises cellulose fibers modified by graft copolymerization in an amount of 5 - 70 Phr (parts by weight per hundred parts of rubber )_; preferably 10 - 40 Phr.

7. A rubber product according to any of claims 5 or 6, characterized in that the cellulose fibers have been modified by graft copolymerization with at least a monomer from the group of C-_j - C-_]2 alkyl acrylate,

C-_j - C-J2 alkenyl acrylate, C-_j - C-J2 alkyl methacrylate or C-_| - C_j2 alkenyl methacrylate, or a copolymer of two or more of said monomers and with an initiator in the form of a Mn^⁺ complex.

8. A rubber product according to any of claims 5 - 7, characterized in that the synthetical rubber is styrene- butadiene rubber.

9. A rubber product according to any of claims 5 - 7, characterized in that the synthetical rubber is nitrile rubber .

10. A rubber product according to any of claims 5 - 7, characterized in that the synthetical rubber is natural rubber .