WO1995026367A1

WO1995026367A1 - Double network elastomers from oriented elastomer network

Info

Publication number: WO1995026367A1
Application number: PCT/US1995/003875
Authority: WO
Inventors: Charles M. Roland; Patrick G. Santangelo
Original assignee: The Government Of The United States Of America, Represented By The Secretary Of The Navy
Priority date: 1994-03-29
Filing date: 1995-03-29
Publication date: 1995-10-05

Abstract

A process for modifying elastomeric structures to improve mechanical properties involves subjecting a pre-cured shape of elastomeric material to an orientation technique to result in an oriented elastomeric material; and curing the oriented elastomeric material while maintaining the orientation of the elastomeric material under conditions effective to result in an elastomeric structure having a predetermined cross-link density and enhanced modulus, the magnitude of orientation being sufficient to result in the enhanced modulus. Typically, the orientation techniques used may include uniaxial extension, biaxial extension, simple shear, planar shear, and inflation. The elastomeric material is capable of strain induced crystallization or incapable of strain-induced crystallization. The enhanced modulus is obtained while at least maintaining failure performance of the elastomeric structure, and the conditions during the curing are sufficient to maintain a constant cross-link density in the double network elastomer while the modulus is enhanced.

Description

DOUBLE NETWORK ELASTOMERS FROM ORIENTED ELASTOMER NETWORK

Background of the Invention

1. Field of the Invention

The present invention relates to the general area of modifying the structure of elastomers in order to obtain improved mechanical properties. More particularly, the present invention is directed to processes for modifying elastomeric structures to improve mechanical properties thereof, such as modulus, that involve subjecting elastomeric material to multiple curing steps that are controlled so as to introduce and apportion cross-links in each curing step to result in predetermined proportions of cross-link densities in the resultant elastomeric material as a result of the respective curing steps. Specifically, the present invention is directed to processes for modifying elastomeric materials that involve subjecting elastomeric material to an initial curing step under controlled conditions effective to result in a pre-cured elastomeric material comprising a first network having a number of cross-links within the range of about 5% to about 35% of the total cross-links of the resultant elastomeric material that has been subjected to another curing step while being subjected to an orientation strain under conditions effective to result in an elastomeric double network having desired total cross¬ links.

2. Description of the Background Art Conventional techniques for increasing the modulus of an elastomer involves increasing its cross-link density. Since increasing the cross-link density of elastomers makes the elastomer more brittle, it is known that higher cross-linking density tend to make the failure properties i.e., tensile strength, crack growth resistance, fatigue life, and tear strength, worse. It is generally accepted, therefore, that with conventional rubber technology, there is a compromise between stiffness and durability. Orientation is an example of a conventional technique for increasing the physical properties of materials comprised of flexible chain polymers. This orientation is thermodynamically unstable, however, and thus can not be exploited above a polymer's glass transition temperature.

It is believed that mechanical properties of elastomers are limited by their intrinsic isotropy. The cross-linking of a rubber produces a set of chain configurations whose equilibrium conditions correspond to a macroscopic state of zero stress. Since rubber is almost always cured while in a relaxed state, a zero stress state after cross-linking equates to a condition of zero stress.

U.S. Patent No. 2,488,188, BALDWIN, discloses that it is advantageous to first cure rubber slightly, then stretch it, followed by finish-curing the rubber while the rubber is in the stretched state to improve certain properties, e.g., tensile strength, elongation, elasticity, and elastic limit. It is disclosed that the invention applies to the curing of various types of natural rubber and unsaturated synthetic rubber polymers, such as chlorobutadiene polymers, copolymers of butadiene and acrylonitrile, copolymers of isobutylene with various diolefins, such as butadiene, isoprene, methyl butadiene, and the like.

Summary of the Invention

Accordingly, it is an object of this invention to provide an elastomeric material with a superior combination of good stiffness and durabilitly. It is another object of the present invention to enhance the cross-link density of an elastomeric material without increasing brittleness.

It is a further object of the present invention to enhance the cross-linking density of an elastomeric material while maintaining the good failure properties of that material.

These and additional objects of the invention are accomplished by a employing two separate and carefully controlled curing steps. In the first curing step, a selected proportion of cross-links are introduced while the elastomer is under substantially zero strain. The resulting pre-cured elastomer is then subjected to a second cross-linking step while oriented by an at least 200% elongation.

Brief Description of the Drawings

A more complete appreciation of the invention will be readily obtained by reference to the following Description of the Preferred Embodiments and the accompanying drawings in which like numerals in different figures represent the same structures or elements, wherein:

Fig. 1 is graph showing the ratio of the double-to-single network moduli at various residual strains.

Fig. 2 is a graph showing the dependence of residual strain on stress strain isotherms.

Fig. 3 is a graph showing tensile strength measured for natural rubber double network samples of various residual strains.

Fig. 4 is a graph showing theoretical ratios of double-to- single network moduli at various residual strains using consecutive equations of Mooney and Rivlin.

Description of the Preferred Embodimen (s)

Typically, in accordance with the present invention, the conditions of curing, such as cure times and temperatures, are adjusted and controlled so that about 5% to about 35%, and more typically between about 5% to about 10%, of the total cross¬ links in the resultant double network elastomer are produced during the initial cure. Subsequently, the pre-cured elastomer is stretched or strained during an orientation to at least 200% elongation, and more typically 600% elongation, and subjected to additional curing under conditions effective to produce a resultant elastomer double network having a desired cross-link density wherein between about 65% to about 95%, and more typically between about 90% to about 95%, of the total cross¬ links in the resultant elastomer double network are introduced during additional curing.

For purposes of the present invention, pre-cured sheets of a rubber, such as natural rubber (NR) , polybutadiene (PBD) , and styrene-butadiene rubber (SBR) , are oriented by an orientation technique, such as uniaxial or biaxial extension, simple or planar shear, inflation, and the like. While maintaining the orientation, the rubber is further cured i.e., more cross links are introduced during the additional curing, to result in the resultant elastomer double network having at least 65% of the total cross-links of the elastomeric material introduced during additional curing.

The final dimensions of the article reflect the relative cross-link densities arising from the two acts of curing, i.e., pre-curing and additional curing, along with the orientation during the second or additional cure. The modulus of the double network elastomer produced in accordance with the present invention is higher than a conventional rubber or single network elastomer having the same total cross-link density.

More specifically, the modulus enhancement of the present invention may be expressed as follows: modulus of double network elastomer modulus enhancement = modulus of single network elastomer having the same cross-link density as the double network elastomer.

The failure properties of the double network elastomer of the present invention are equivalent to the conventional rubber or single network elastomer when no crystallization takes place. On the other hand, when the deformation giving rise to the failure is non-relaxing of strain-induced crystallization e.g. , NR. high cis 1,4-PBD, polyisobutylene (PIB) , the resistance to failure of the double network elastomer of the present invention is better than that of the conventional rubber or single network elastomer having the same total cross-link density. In contrast to conventional rubber technology that forces a trade-off between stiffness and strength, modulus and durability, the present invention enables higher modulus to be realized while simultaneously maintaining or improving failure performance.

Prior to the present invention, no method for modifying the structure of elastomers was known that i) increases the modulus, (ii) maintains a constant cross-link density, and

(iii) maintains or improves failure properties of elastomers. In accordance, with the present invention, rubber is cured while unrelaxed, e.g., while stretched, such that the subsequent state of elastic equilibrium is shifted away from zero strain. The resultant double network elastomer structure provides thermodynamically stable orientation, which significantly alters the mechanical properties of an elastomer.

In accordance with the present invention, the failure properties are maintained by keeping a constant cross-link density, while the modulus is enhanced by double network formation. In addition, for elastomers and rubbers which strain crystallize, an enhancement of strain crystallizability can be obtained by virtue of the double network process. This leads to significantly better failure properties, since the strain crystallization is the primary mechanism for failure resistance.

The process of the present invention involves the following steps: a) curing elastomeric material under conditions of substantially zero strain, time and temperature to result in a pre-cured elastomeric network comprising less than about 40%, and more typically less than about 35%, of total cross-links that are present in the resultant elastomeric double network structure; b) subjecting the pre-cured elastomeric material to an orientation technique to result in an oriented elastomeric material having at least about 200% elongation; and c) curing the oriented elastomeric material while maintaining such orientation under conditions effective to result in a resultant elastomeric structure comprising a predetermined cross-link density of at least about 60%, and more typically at least about 65%, of total cross-links present in the resultant elastomeric double network structure, wherein the resultant elastomeric double network structure exhibits enhanced modulus.

In accordance with the present invention, the cure times and temperatures are adjusted and controlled so that most typically between about 5% to about 35%, and most often between about 5% to about 10%, of the total cross-links of the resultant elastomeric double network structure are produced during initial cure at zero strain. Subsequently the pre-cured elastomer is stretched during an orientation to at least about 200% elongation, and more typically 600% elongation, and cured to produce a resultant elastomer double network having between about 65% to about 95% and more typically between about 90% to about 95% of the total cross-links.

In accordance with the present invention, conditions during the curing steps are effective to result in magnitude of orientation sufficient to result in enhanced modulus.

For purposes of the present invention, suitable methods of cross-linking include essentially all conventional curing methods, i.e., sulfur vulcanization, efficient vulcanization

(also known as sulfurless or "EV" curing) , peroxide curing (using any of the variety of commercial peroxides such as dicumyl peroxide) , electron irradiation, and gamma ray irradiation. Examples of specific cross-linking methods suitable for specific elastomers include: for neoprenes, metal oxides; phenolformaldehyde resins for butyl rubber; and for butyl and other unsaturated rubbers, p-benzoquinone dioxime.

For purposes of the present invention, achieving a desired proportioning of the total cross-link density is accomplished in different manners depending on the particular method of cross-linking. Typically, for chemical methods, time and temperature are the primary variables used to control the cross-link apportionment. Radiation curing would rely on dose,

(i.e., radiation intensity and exposure time). Examples of more typical curing systems suitable for purposes of the present invention include curing systems selected from the group consisting of sulfur vulcanization, peroxide curing, and radiative curing.

Sulfur vulcanization typically has two stages. The first stage, referred to as the induction or scorch period, involves the curing agents reacting with themselves prior to actual cross-linking reaction. When adjusting for cross-link apportionment, the time of the second stage, where cross-links are actually formed, is important. The length of time for each stage of the process is dependent on the temperature and the amount and type of curatives used. Thus, as a result of the scorch period, at a given cure temperature, it is possible for the first cross-link density of a soluble network rubber to have a smaller percentage of the total cross-links even while experiencing a longer cure time than the second cross-linking period.

Peroxide curing involves the formation of thermal decomposition of an organic peroxide. These react with the elastomer to produce polymer radicals which combine to form a cross-link. Therefore, the rate of cross-linking is dependent on the rates of peroxide decomposition and polymer radical production.

Radiative puring typically also proceeds via radical mechanisms. The polymer radicals produced through the use of energizing radiation combine to produce a cross-link. The number of cross-links produced by this technique is a function of the duration and intensity of radiation used.

In accordance with the present invention, orientation is accomplished using orientation techniques. For purposes of the present invention, typical orientation techniques include those selected from the group of orientation procedures comprising uniaxial extension, biaxial extension, simple shear, planar shear, and inflation.

For purposes of the present invention, the strains at which the second cross-linking is to be introduced into the elastomeric double network is specified to be in the range from 20% to 600% elongation. Below 20% elongation, there are no beneficial effects on properties as compared to conventional rubbers. More typically, however, the lower limit of elongation is about 200% elongation.

The upper limit for the cross-linking strain is in principle the material's maximum extensibility; however, in practice this yields an overly large "rejection rate", that is, many samples fail during their production. For practical reasons, therefore, the strain during cross-linking is more typically limited to not greater than 600%.

The number of cross-links generated in the first network is specified to be in the range from about 5% to about 35% of the total cross-links introduced into the resultant double network material. If less than 5% of the total cross-links ultimately present in the final resultant double network are introduced during the initial stage of cross-linking in forming the first network, then the material will not have the coherent strength to survive the second stage of the cross-linking process. No beneficial effects are realized in double network material if the number of cross-links introduced into the first network exceeds about 35 % of the total number of cross-links in the resultant double network elastomer.

For purposes of the present invention, conventional procedures for the measurement of cross-link density are used. Examples of conventional cross-link measurement procedures include (i) determination of the degree of swelling in a solvent, and (ii) determining the equilibrium (fully-relaxed) modulus.

For purposes of the present invention suitable elastomeric material includes members selected from the group consisting of elastomers not capable of strain induced crystallization, and elastomers capable of strain-induced crystallization, wherein elastomeric materials not capable of strain-induced crystallization comprise at least one member selected from the group consisting of unsaturated synthetic rubber polymers, copolymers of butadiene and styrene, copolymers of butadiene and acrylonitrile, and elastomers capable of strain induced crystallization include members selected from the group consisting of natural rubber (NR) polybutadiene (PBD) , styrene butadiene (SBR), polyisobutylene (PIB), and high cis 1,4 - polybutadiene (high cis 1,4 - PBR) .

In accordance with the present invention, the conditions during curing are sufficient to maintain a constant cross link density in said double network elastomer while the modulus is enhanced, and while at least failure performance of the elastomeric structure is maintained.

In accordance with the present invention, it has been unexpectedly discovered that there is little difference in the fatigue lifetime of a rubber incapable of strain crystallizing, by virtue of its chemical structure, such as SBR, nitrile rubber, and the like, compared in a fully relaxing deformation cycle to a non-relaxing deformation cycle. Contrarily, a rubber capable of strain crystallization, e.g., natural rubber, neoprene, and the like, will have a markedly longer fatigue life for non-relaxing deformations as compared to fully relaxing deformations.

Thus, the present invention, in part, is based on the discovery that one can take advantage of double network elastomers and attain non-relaxing deformation levels of performance even when they are subjected to fully relaxing deformation cycles. This is a result of the intrinsic orientation of the double network rubber. Accordingly, elastomeric or rubber parts and devices may be designed and developed without the constraint of having to avoid fully relaxing deformations. Furthermore, applications for which fully relaxing conditions are unavoidable can obviously benefit from the use of double network technology in accordance with the present invention.

Having described the invention, the following examples are given to illustrate specific applications of the invention including the best mode now known to perform the invention. These specific examples are not intended to limit the scope of the invention described in this application.

Example I The following example was conducted to demonstrate aspects of the present invention. A. Natural Rubber with Peroxide Cure Company

SMR-L 100 Peroxide 2 R . T .

Vanderbilt

(Varox DCP-R)

Procedure:

1) 1st Cure: The rubber was cured in a standard fatigue mold using a 12 ton press at 120^βC for one hour.

2) 2nd Cure: After the first cure, the sheet of rubber was cut into 2.5" x .5" wide strips. The rubber strips were stretched to various lengths and cured in a vacuum oven

(<30 in Hg) at 160°C for 1.5 hours.

In this example, curing was weighted towards the second network, so that more cross-links were formed in the second network than were formed during the second cure. In Fig. 1 the ratio of double-to single-network moduli has been plotted at various residual strains. The data is taken from Fig. 2 at an extension ratio of 1.1. In Fig. 2, dependence of residual strain on stress-strain isotherms is

« shown at 23^βC for natural rubbers where the first cross-linking step is achieved in an unstrained state, and the second cross- linking step is completed at various strains. For the purpose of Fig. 2, the concentration of cross-links produced during the first and second cross-linking steps was maintained constant (residual strain :1.0(*); 1.4(^»); and 2.3 (•)). For purposes of Fig. 2, uniaxial tension measurements were made on the samples at room temperature. The strain of test specimens was recorded at various applied stresses, and the data was taken after the samples had reached mechanical equilibrium, i.e., the strain was no longer changing at a given stress. The double-network rubbers and the single- network rubber, i.e., residual strain of 1.0 have the same total cross-link density. In Fig. 3, the tensile strength measured for natural rubber double-networks samples of various residual strains has been plotted on the graph. Also shown is the tensile strength of a single-network rubber (residual strain of 1.0) having the same total cross-link density. The double-networks posses an elevated modulus while maintaining strength. B. Peroxide Cure System:

Natural Rubber 100 parts

Peroxide 2 parts

* Varox PCP-R from Vanderbilt Chem. Co.

1) First Cure: 60 minutes at 120°C

2) Second Cure: 90 minutes at 160°C while strained to indicated level.

Strain Durinα Residual Strain Modulus

Second Cure

0% 0% 1.0

60% 44% 0.85

130% 97% 0.85

200% 129% 0.7

310% 199% 1.1

390% ' 265% 1.5

480% 290% 1.7

575% 330% 1.9

This data illustrates that the modulus of a double network can be less than or greater than that of a single network of equal cross-link density, depending on the cross- linking strain and the cross-link distribution.

Example II The following example was conducted to demonstrate aspects of the present invention. In this Example II, unlike Example I, the cure distribution was not weighted towards the second network.

Natural Rubber with Fast Cure Company SMR-L 100 Stearic Acid 2 Fisher

ZnO 5 Fisher

Age-Rite resin D 1 R . T

Vanderbilt (Antioxidant)

Butyl Eight 3

Vanderbilt (Ultra-Accelerator)

Altax 0.5

Vanderbilt

(Accelerator)

Sulfur l Monsanto

Procedure:

1) 1st Cure: The rubber was cured in a standard fatigue mold using a 12 ton press at

80°C for one hour.

2) 2nd Cure: After the first cure, the sheet of rubber was cut into 1.5" long x .5" wide strips. The rubber strips were stretched to various lengths and cured in an vacuum oven (<30 in Hg) at 80^βC for 1.0 hours. Strain Durinq-2nd X Linking Residual Strain Modulus 0% 0% 1.72 75% 25% 1.68

163% 48% 1.57

294% 69% 1.57

394% 78% 1.88

Example III The following example was conducted to demonstrate aspects of the present invention using a: Sulfurless Cure System. Natural Rubber 100 parts N2 34 Black 55 parts

Zinc Oxide ^• 5.0 parts

Stearic Acid 0.5 parts

Pine Tar 5.0 parts Anti-Oxidant 1.0 parts

Morpholino -2 - 3.5 parts

Benzonthiazole Disulfide parts

Procedure

1) First Cure: 15 minutes at 145°C 2) Second Cure: Stretch to 400% the 32 minutes at 145^βC

Results: double network rubber has modules = 5.7 MPa.

Control: (all curing done at zero-strain) has modules = 4.4 MPa. Example IV

The following example was conducted to demonstrate aspects of the present invention using a:

Sulfur Cure System

Polyisoprene* 100 Zinc Oxide 5.0

Stearic Acid 3.0

TBBS 0.75

Sulfur 2.4

♦Synthetic Rubber from Goodyear "Natsyn 2200" N-t-butyl-2-benzothiazole sulferamide

1) First Cure: 35 minutes at 140°C

2) Second Cure: 15 minutes at 140°C while strain = 600%.

Results: double network modulus was 1.5 x In Fig. 4, the theoretical ratio of double-to single- network moduli at various residual strains using the constitutive equations of Mooney and Rivlin are depicted on the graph. • For purposes of Fig. 3, constitutive equations were used to model the stress/strain behavior of rubbers; these equations were fitted to experimental data then used to make predictions as shown in Fig. 4. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

ClaimsWhat is claimed is:

1. A process for modifying elastomeric structures to improve mechanical properties, said process comprising: a) curing elastomeric material under substantially zero strain and conditions effective to result in a pre-cured elastomeric network having cross-links; b) subjecting said pre-cured elastomeric network to orientation to result in an oriented elastomeric network having at least about 200% elongation; and c) curing said oriented elastomeric network while maintaining said orientation under conditions effective to result in an elastomeric structure, thus forming a modified elastomeric structure, wherein said curing in a) provides up to about 35% of the total number of cross-links in said elastomeric structure.

2. The process of claim 1, wherein said precured elastomeric material has between about 5% to about 35% of the total number' of cross-links in said modified elastomeric structure.

3. The process of claim 2, wherein said precured elastomeric material provides between about 5% to about 10% of the total number of cross-links in said modified elastomeric structure.

4. The process of claim 1, wherein curing is effected by a curing system selected from the group of elastomer curing systems consisting of sulfur vulcanization, peroxide curing, and radiative curing.

5. The process of claim 1, wherein said orientation technique is selected from the group of procedures consisting of uniaxial extension, biaxial extension, simple shear, planar shear, and inflation.

6. The process of claim 5, wherein said orientation technique is uniaxial extension.

7. The process of claim 6, wherein said elastomeric material comprises at least one elastomer not capable of strain induced crystallization.

8. The process of claim 6, wherein said orientation comprises at least one deformation cycle.

9. The process of claim 6, wherein said elastomeric material comprises elastomers capable of strain induced crystallization.

10. The process of claim 9, wherein said orientation comprises a non-relaxing deformation cycle.

11. The process of claim 10, wherein said elastomers capable of strain induced crystallization are members selected from the group consisting of natural rubber (NR) , polybutadiene (PBD) , styrene butadiene (SBR) , polyisobutylene (PIB) , and high cis 1,4 - polybutadiene (high cis 1,4 - PBR) .

12. The process of claim 8, wherein said elastomeric material comprises elastomers not capable of strain crystallization.

13. The process of claim 12, wherein said orientation comprises a relaxing deformation cycle.

14. The process of claim 13, wherein said relaxing deformation cycle is a fully relaxing deformation cycle.

15. The process of claim 13, wherein said elastomeric materials not capable of strain-induced crystallization comprise at least one member selected from the group consisting of unsaturated synthetic rubber polymers, copolymers of butadiene and styrene, copolymers of butadiene and acrylonitrile.

16. The product of the process of claim 1.

17. The product of the process of claim 2.

18. The product of the process of claim 9.

19. The product of the process of claim 11.

20. The product of the process of claim 15.