METHOD OF TREATING UNCURED THERMOSETTING RESIN MATRICES

Abstract

Treating an uncured resin to impart energy to the resin enables a stable Tg to be achieved whereby the resin may be stored and has a low tack enabling subsequent processing and handling. The invention provides a method of achieving a stable Tg without the resin starting to cure and a method of determining the treatment regime by which a resin with a stable Tg may be obtained. The resin may be fresh or reused uncured resin and may contain fibrous reinforcement.

Claims

1. A method of producing a stabilised uncured resin matrix composition comprising providing an uncured resin matrix composition having an initial glass transition temperature (Tg) and subjecting the resin matrix composition to: i. a first regime comprising imparting energy to the uncured resin matrix composition to raise the Tg by at least 5% to provide a raised Tg whereby the residual cure of the resin matrix is reduced by a maximum of 20%, preferably a maximum of 10% and more preferably a maximum of 5%; and ii. a second regime comprising storing the uncured resin matrix composition to provide a stabilised uncured resin matrix composition wherein the Tg of the stabilised uncured resin matrix composition is such that it does not increase by more than 10% from the raised Tg when stored for 14 days.

2. A method according to claim 1 wherein the uncured resin matrix composition is initially subjected to a method of determining the treatment regime for stabilising the Tg of the uncured resin matrix composition comprising: i) providing an uncured resin matrix composition having an initial Tg and selecting a first regime comprising an elevated temperature to impart energy to the uncured resin matrix composition whereby the residual cure of the resin matrix is reduced by a maximum of 20%, preferably a maximum of 10% and more preferably a maximum of 5%; ii) imparting energy to the resin matrix composition in the first regime so as to increase the Tg of the resin matrix composition; iii) selecting a second regime comprising storing the resin matrix composition wherein the second regime is different to the first regime; iv) subjecting the resin matrix composition to the second regime whereby the residual cure of the resin matrix is reduced by a maximum of 10% and more preferably a maximum of 5% the resin matrix composition; v) measuring the raised Tg of the resin matrix composition prior to or at commencement of the second regime and measuring the Tg of the resin matrix composition periodically until over 14 days either: i. the periodically measured Tg does not increase by more than 10% from the raised Tg thereby determining the conditions of the first regime and the second regime; or ii. where the periodically measured Tg increases by more than 10% from the raised Tg, modifying the first regime and/or second regime and repeating steps ii), iv) and v) until the periodically measured Tg does not increase by more than 10% from the raised Tg.

3. A method according to claim 1 wherein the resin matrix composition comprises polymer chains and imparting energy to the uncured resin matrix composition provides a reduction in cure enthalpy to complete cure which is in the range of from 2% to 20%.

4. A method according to claim 1 wherein the raised Tg is increased by at least 5% from the initial Tg.

5. A method according to claim 1 wherein the raised Tg is increased by at least 25% from the initial Tg.

6. A method according to claim 1 wherein the temperature in the second regime is ambient.

7. A method according to claim 1 wherein energy is imparted to the uncured resin matrix composition in the first regime by heating the uncured resin matrix composition to an elevated temperature for a first period of time and subjecting the resin matrix composition to pressure.

8. A method according to claim 1 wherein energy is imparted to the uncured resin matrix composition in the first regime for a period of 1 to 20 hours.

9. A method according to claim 1 wherein the second regime comprises storing the resin matrix composition for at least 12 hours at ambient temperature.

10. A method according to claim 1 wherein the second regime comprises storing the resin matrix composition for a period of at least 24 hours to 10 weeks at ambient temperature.

11. A method according to claim 1 wherein the uncured resin matrix composition is impregnated in a fibrous reinforcement material.

12. A method according to claim 14 wherein the fibrous reinforcement material pre-impregnated with an uncured resin matrix composition comprises an unused or uncured composite.

13. A method according to claim 1 wherein the uncured resin matrix composition is in the form of discrete elements.

14. A method according to claim 1 wherein the stabilised uncured resin matrix composition comprises discrete elements having a tack in the range of from 0.1 to 0.6 as measured using the method as disclosed in the description of this application.

15. A method of determining a treatment regime for stabilising the glass transition temperature (Tg) of a curable resin matrix composition comprising: i. providing a resin matrix composition wherein the resin matrix composition may be uncured having an initial Tg and selecting a first regime comprising an elevated temperature to impart energy to the uncured resin matrix composition whereby the residual cure of the resin matrix is reduced by a maximum of 20%, preferably a maximum of 10% and more preferably a maximum of 5%; ii. imparting energy to the resin matrix composition in the first regime so as to increase the Tg of the resin matrix composition; iii. selecting a second regime comprising storing the resin matrix composition wherein the second regime is different to the first regime; iv. subjecting the resin matrix composition to the second regime whereby the residual cure of the resin matrix is reduced by a maximum of 10%; vi) measuring the Tg of the resin matrix composition periodically until over a selected period of time ranging from 12 hours to 144 hours or periods thereof either: a. the periodically measured Tg does not increase by more than 1%, or 2%, or 4%, or 5% or 10% from the raised Tg thereby determining the conditions of the first regime and the second regime; or b. where the periodically measured Tg increases by more than 1%, or 2%, or 4%/o, or 5% or 10% from the raised Tg, modifying the first regime and/or second regime and repeating steps ii), iv) and v) until the periodically measured Tg does not increase by more than 1%, or 2%, or 4%, or 5% or 10% from the raised Tg.

16. A treated uncured or unused resin matrix composition having a stable glass transition temperature (Tg) at ambient temperature wherein the uncured resin matrix composition has a raised Tg after being treated and the Tg of the treated uncured resin matrix composition remains within 10% of the initial raised Tg when stored for at least 1 day.

17. A treated uncured resin matrix composition according to claim 16 wherein the Tg of the treated uncured resin matrix composition remains within 10% of the raised Tg when stored for at least 1 week.

18. A treated uncured resin matrix composition according to claim 16 wherein the Tg of the treated uncured resin matrix composition remains within 10% of the raised Tg when stored for at least 1 month.

19. A treated uncured resin matrix composition according to claim 16 in the form of discrete elements.

20. A treated uncured resin matrix composition according to claim 19 wherein the discrete elements have a tack F/Fref of not more than 0.1 to 0.45 at ambient temperature where Fref=28.19N and F is the maximum debonding force.

21. (canceled)

22. (canceled)

Description

[0066] The invention is illustrated by the following non-limiting examples and with reference to the accompanying drawings in which

[0067] FIG. 1 presents a diagrammatic view of a lay-up of a resin matrix composition according to an embodiment of the invention using a vacuum bag;

[0068] FIG. 2 presents a diagrammatic view of gripping layers of a sample according to another embodiment of the invention; and

[0069] FIG. 3 presents a diagram showing the relationship between peel torque (in N/10 mm) in relation to Tg (in C.).

EXAMPLE 1

[0070] Several batches of pre-preg products comprising uncured resin matrix composition and carbon fibres were subjected to T Peel testing. The pre-preg product was as follows:

[0071] 3 batches of M21E/34%/UD 194/IMA-12K: [0072] Batch 1: N23C14 02A [0073] Batch 2: N14302 12A [0074] Batch 3: N14205 05A

[0075] 1 batch of M21E/34%/UD 268/IMA-12K: [0076] Batch 1*: N22A1801A

[0077] Stabilizing T.sub.g

[0078] Five samples from each batch were subjected to a first regime by heating in an oven at 50 C. under a dry atmosphere for following periods: 24 h, 72 h, 144 h, 152 h and 200 h. For each batch, a sample of fresh material was kept without being subjected to the first regime. And referred to as T0 below. After the first regime, the samples were held in a freezer at 18 C. (0 F.). Furthermore, each specimen was analyzed by DSC.sup.1 test just after the first regime and prior to the second regime to determine Tg. The Tg results are shown in Table 1. .sup.1Differential scanning calorimetry

[0079] The DSC cycle used for analysis consisted of a ramp of 10 C./min from 60 C. (76 F.) to 315 C. (600 F.); samples comprised 7 mg of resin matrix composition (approximately 20 mg of prepreg). The Tg value measured is the midpoint temperature. Unless otherwise stated, DSC measurements may be carried out using this method.

TABLE-US-00001 TABLE 1 Tg for each batch T0 24 h @ 50 C. 72 h @ 50 C. 144 h @50 C. 152 h@50 C. 200 h@50 C. BATCH 1 0.99 C. 1.1 C. 7.39 C. 17.63 C. 18.19 C. 29.74 C. BATCH 2 2 C. 0.97 C. 6.24 C. 16.6 C. 19.33 C. 26.13 C. BATCH 3 0.84 C. 2.91 C. 9.19 C. 15.41 C. 23.04 C. 27.39 C. BATCH 1* 0.24 C. 2.63 C. 8.15 C. 17.41 C. 19.71 C. 27.91 C.

Storage Simulation

[0080] The samples were subjected to a process to simulate storage of resin matrix composition chips in a bulk quantity in a storage bag under harsh conditions of storage. This simulation increased the chance that clustering or agglomeration of chips might occur A temperature of 60 C. to mimic bright sunlight and a pressure of 1 bar (14.5 psi) was applied, corresponding to the higher pressure that a chip could perceive at the bottom of a storage bag. The storage conditions were performed by using a vacuum bag method as set out below and with reference to FIG. 1.

[0081] The apparatus for the vacuum bag method is shown in FIG. 1 which shows a tool 1 upon which is mounted a vacuum bag 2 having a first side 2a and a second side 2b sealed with mastic 2c and a sandwich arrangement comprising two layers of Teflon coated glass, referred to as breathers, 3a, 3b, layers of resin matrix composition product 4a and 4b and a Teflon coated glass primer 5 (length 50 mm) with which to commence the T Peel test.

[0082] Two layers of the sample resin matrix composition are mounted in the apparatus and the assembly was placed in an oven for 15 hours at 60 C. and a pressure of 1 bar was applied using a vacuum pump.

[0083] Samples Preparation and T Peel Test (in Accordance with ASTM D1876)

[0084] Once the samples had been subjected to the Storage Simulation the samples were prepared for use in T Peel tests. The T Peel test allows the adhesion strength between two layers of a sample bonded to each other to be determined using a tensile machine. An average strength of debonding between layers is measured and a Peel torque is determined. The Peel torque is equal to the average strength normalized to a 10 mm width. For each batch and each treatment, 3 samples were tested according to T Peel test ISO11339. The samples were cut to the following dimensions: [0085] Width: 30 mm [0086] Length: 300 mm [0087] Depth: 2 plies [0088] Orientation: 0

[0089] The two layers of the sample were gripped respectively in a fixed jaw and a mobile jaw as shown in FIG. 2. The two layers of the sample are then separated by moving the mobile jaw away from the fixed jaw in the direction shown. The force applied in the test to separate the layers and its variation is measured using software in accordance with ASTM D1876, from which an average value of debonding strength is calculated. The Peel torque value is determined by the climbing drum peel test to determine the peel resistance of adhesive bonds in accordance with ASTM D1781. This test consists of peeling a thin strip of metal from a thick strip of resin matrix material by winding the thin strip around a drum. Torque is applied to the drum by pulling down on straps wrapped around the drum. The thin strip of metal is wound on the drum at a smaller radius than the straps. The difference in radius (i.e. moment arm) results in a large torque being applied to the drum compared with that applied on the thin strip. The resultant upward motion causes the thin strip to peel from the thicker strip. The average peel torque T can be calculated as follows:

T=(RoRi)(FpFo)/b

where Ro is the flange radius, Ri is the drum radius, Fp is the average load required to bend and peel adherend (including load required to overcome the torque resistance of the drum), Fo is the load required to overcome the torque resistance of the drum and b is the specimen width. Both Ro and Ri account for one half the loading strap thickness. The upward motion of the drum causes the thin strip to be peeled from the thicker resin matrix material resulting in bond failure.

[0090] The results of the T Peel tests are set out in Tables 2 to 5. For each batch three samples were tested under each regime and the results are the mean value of the three tests.

TABLE-US-00002 TABLE 2 Average deboning Average Peel BATCH 1 Strength .sup.2(N) torque .sup.3(N/10 mm) T0 34.8 11.6 24 h @ 50 C. 38.7 12.9 72 h @ 50 C. 2.2 0.7 144 h @50 C. 0.1 0.0 152 h@50 C. 0.4 0.2 200 h@50 C. 0.2 0.1

TABLE-US-00003 TABLE 3 Average deboning Average Peel BATCH 2 Strength .sup.4(N) torque .sup.5(N/10 mm) T0 34 11.3 24 h @ 50 C. 43.2 14.4 72 h @ 50 C. 2.1 0.7 144 h @50 C. 0.4 0.1 152 h@50 C. 0.2 0.1 200 h@50 C. 0 0

TABLE-US-00004 TABLE 4 Average deboning Average Peel BATCH 3 Strength .sup.6(N) torque .sup.7(N/10 mm) T0 40.2 13.4 24 h @ 50 C. 47.1 15.7 72 h @ 50 C. 1.5 0.5 144 h @50 C. 0.9 0.3 152 h@50 C. 0.2 0.1 200 h@50 C. 0.3 0.1

TABLE-US-00005 TABLE 5 Average deboning Average Peel BATCH 4 Strength .sup.8(N) torque .sup.9(N/10 mm) T0 42.0 14.0 24 h @ 50 C. 41.4 13.8 72 h @ 50 C. 15.5 5.1 144 h @50 C. 1.7 0.6 152 h@50 C. 1.6 0.5 200 h@50 C. 1.4 0.5

[0091] A plot of the T Peel value versus Tg midpoint value for each batch was plotted and is shown in FIG. 3. .sup.2Mean value calculated from 3 specimens results.sup.3Mean value calculated from 3 specimens results.sup.4Mean value calculated from 3 specimens results.sup.5Mean value calculated from 3 specimens results

[0092] For batches 1, 2 and 3 adhesion decreases significantly around 5 C. of Tg and debonding occurs at approximately 15 C. A slight difference is noticed with results of batch 1* in that the adhesion decreases less abruptly than with the other batches and debonding occurs at around a Tg of 20 C. The batch 1* plies were thicker than the batches 1, 2 and 3 plies which may contribute to the tailing effect observed for batch 1*. .sup.6Mean value calculated from 3 specimens results.sup.7Mean value calculated from 3 specimens results.sup.8Mean value calculated from 3 specimens results.sup.9Mean value calculated from 3 specimens results

[0093] For every batch debonding is reduced where the Tg is 5 C. and debonding occurs completely where the Tg value is above 20 C. for chips which have been treated according to the method of the invention and subjected to a simulation of harsh storage conditions.

[0094] In summary, a resin matrix composition material which is staged so that it has a Tg over 20 C. is suitable for storage over an extended period at ambient temperature without exhibiting clustering or agglomeration irrespective of whether the storage temperature is regulated.