Method for estimating resin life and method for selecting resin

Abstract

To provide a resin life estimation method and a resin selection method which can estimate the life of a resin at arbitrary temperature and humidity.SOLUTION: A resin life estimation method includes the steps of: measuring a relation between an exothermic peak temperature measured using a differential scanning calorimeter of a resin at a constant relative humidity RH1 and a load time, at a plurality of temperatures T, calculating a time L until the resin in each of the temperatures T reaches the end of the life, and calculating activation energy Ea from a relation between the temperature T and the time L; measuring a relation between an exothermic peak temperature of the resin at a constant temperature T1 and the load temperature, at a plurality of relative humidities RH, calculating a time L until the resin at each of the relative humidities RH reaches the end of the life, and calculating a humidity acceleration factor n from a relation between the relative humidity RH and the time L; calculating a time LA until the resin reaches the end of the life under an acceleration condition of a temperature TA and relative humidity RHA; and calculating a time LN until the resin reaches the end of the life under a standard condition of a temperature TN and a relative humidity RHN from Expression 2.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for estimating the lifespan of a hydrolyzable resin under arbitrary environmental conditions, and to a method for selecting a resin. [Background technology]

[0002] Among resins, elastomers, for example, those having ester, urethane, or amide groups in their main chain, are known to deteriorate over time due to hydrolysis. The main parameters contributing to hydrolysis are temperature and humidity (relative humidity).

[0003] As a method for estimating the degradation of resins due to hydrolysis, it is known to focus on the temperature dependence of the reaction and use the Arrhenius model to estimate the degree of resin degradation, as shown in Patent Document 1, for example. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-170989 [Overview of the project] [Problems that the invention aims to solve]

[0005] The estimation method described in Patent Document 1 estimates the current degree of resin degradation, but it is difficult to estimate the future degree of degradation, i.e., the lifespan of the resin. Furthermore, the estimation method in Patent Document 1 only considers temperature and does not consider humidity. For this reason, there is a need for a method to estimate the lifespan of hydrolyzable polyester resins, polyurethane resins, polyamide resins, or blends thereof with elastomers or other resins used in medical devices, at arbitrary temperatures and humidity levels.

[0006] The present invention was made to solve the above-mentioned problems, and aims to provide a resin life estimation method and a resin selection method that can estimate the lifespan of a resin at any temperature and humidity. [Means for solving the problem]

[0007] The present invention (1) Resin Life Estimation Method for achieving the above objective is a resin life estimation method for estimating the time it takes for a hydrolyzable resin to reach its lifespan at an arbitrary temperature T and relative humidity RH, wherein the relationship between the exothermic peak temperature and load time, measured using a differential scanning calorimeter of the resin at a constant relative humidity RH1, is measured at multiple temperatures T, the time L until the resin reaches its lifespan at each temperature T is calculated, and the activation energy E is derived from the relationship between the temperature T and the time L. a The process involves calculating the following: measuring the relationship between the heat-generating peak temperature of the resin at a constant temperature T1 and the load time at multiple relative humidity RHs, calculating the time L until the resin reaches the end of its lifespan at each relative humidity RH, and calculating the humidity acceleration coefficient n from the relationship between the relative humidity RH and the time L; and temperature T A and relative humidity RH A The time L until the resin reaches the end of its lifespan under the acceleration conditions A The process of calculating the Boltzmann constant, with temperature T being k. N and relative humidity RH N The time L until the resin reaches the end of its lifespan under standard conditions N The process includes the step of calculating from the following formula. JPEG0007923200000001.jpg22100 [Effects of the Invention]

[0008] The resin life estimation method configured as described above can accurately calculate the activation energy and humidity acceleration coefficient by measuring the exothermic peak temperature using a differential scanning calorimeter, thereby enabling highly accurate estimation of the resin life under arbitrary temperature and relative humidity conditions.

[0009] (2) In the resin life estimation method of (1) above, the resin may be a resin material used for medical devices. This allows the service life of a medical device to be appropriately set.

[0010] (3) The resin selection method according to the present invention comprises preparing a plurality of types of resins, estimating the time until each resin reaches its end of life by any one of the above resin life estimation methods, and selecting a resin based on the time until the resin reaches its end of life. The resin selection method configured as described above enables appropriate selection of a resin based on the assumed life of the resin. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] [Figure 1] It is a graph showing the relationship between exothermic peak temperature and loaded days for a sample resin. [Figure 2] It is a graph showing the relationship between degradation rate and loading time for a sample resin. [Figure 3] It is a graph showing the relationship between degradation rate and loading time for a sample resin when relative humidity is fixed and temperature conditions are changed. [Figure 4] It is a graph showing the relationship between the life of a sample resin when relative humidity is fixed and the reciprocal of temperature. [Figure 5] It is a graph showing the relationship between degradation rate and loading time for a sample resin when temperature is fixed and relative humidity conditions are changed. [Figure 6] It is a graph showing the relationship between the life of a sample resin and relative humidity when temperature is fixed. MODE FOR CARRYING OUT THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The resin life estimation method according to the present embodiment targets resin materials used in medical devices such as catheters. However, the target resin may be one used for applications other than medical devices, and the application is not particularly limited. The resin targeted by the resin life estimation method is a polyester resin, polyurethane resin, polyamide resin or elastomer having ester, urethane, or amide groups in the main chain, and is a resin that degrades due to hydrolysis. Specific examples include polyester elastomers, urethane elastomers, and nylon elastomers. However, the resin targeted by the resin life estimation method is not limited to these, and any resin that degrades due to hydrolysis can be targeted by the resin life estimation method.

[0013] When performing life estimation, a model that can take into account hydrolysis, which is the degradation mechanism, is required. The parameters contributing to the progress of hydrolysis are mainly "humidity" and "temperature". The Eyring model is used as a life estimation model that can consider these two parameters. The general formula of the Eyring model is shown in Mathematical Formula 1.

[0014]

Number

[0015] L is the time to reach a certain (cumulative) failure probability (hereinafter referred to as failure time). In life estimation, the failure time L under accelerated conditions A is used to calculate the failure time L under standard environment N , and for this purpose, it is necessary to obtain the acceleration factor A. Obtaining the acceleration factor A from Mathematical Formula 1 gives Mathematical Formula 2.

[0016]

Number

[0017] In the present embodiment, Mathematical Formula 2 is used to estimate the failure time in a standard environment, that is, the life. Note that the activation energy (E aSince the humidity acceleration coefficient (n) needs to be calculated experimentally, we will first derive these two values.

[0018] To estimate lifespan, it is necessary to define lifespan and its scale. In this embodiment, the calculation is performed by applying a reliability analysis method based on failure rate. Failure rate is mainly defined as "the percentage of failures that occur per unit time for all samples that are functioning normally from the start of use until time t has elapsed." This failure rate decreases, increases (or remains constant) over time. Taking components such as semiconductor devices as an example, it decreases during the period when initial defects occur, becomes constant during the stable period (time when random failures occur), and shows an increasing trend during the time when wear-out failures occur.

[0019] However, since the hydrolysis of the resin in question is an irreversible phenomenon, the degree of degradation will not decrease. In other words, the degree of degradation will increase over time, and it is expected to follow a trend similar to that of wear failure occurrence (increasing failure rate type).

[0020] Based on the above results, in this embodiment, the degree of hydrolysis is expressed as a value from 0 (no hydrolysis) to 100 (hydrolysis has stopped), and life estimation is performed by applying a reliability analysis method.

[0021] Next, we determine a value that indicates the degree of hydrolysis of the resin assumed above. For this value, we use the exothermic peak temperature measured by a DSC (Differential Scanning Calorimeter). It has been found that the exothermic peak temperature shifts to a higher temperature in proportion to the amount of degradation by-products produced by hydrolysis. In other words, by setting the exothermic peak temperature when the resin is undegraded as the zero point and the exothermic peak temperature when the resin has degraded to its limit (when hydrolysis has stopped) as the upper limit (=100%), it becomes possible to express the degree of hydrolysis as a value from 0 to 100 (%).

[0022] As shown in Figure 1, the exothermic peak temperature of the resin measured by DSC can be seen to increase in proportion to the number of days of loading. Therefore, the degree of hydrolysis can be accurately measured using the exothermic peak temperature, enabling highly accurate life estimation.

[0023] Hydrolysis stops when most of the decomposable bonds, such as esters, urethanes, and amide groups, have been broken down. To confirm that the shift in the exothermic peak temperature also stops or does not fluctuate further once the decomposition is complete, a long-term load test was conducted under 80°C and 95% RH conditions. The results are shown in Table 1.

[0024] [Table 1]

[0025] The results in Table 1 confirm that the shift in the exothermic peak temperature has stopped, as the exothermic peak temperature no longer changes.

[0026] For estimation purposes, the peak exothermic temperature must correlate with the degree of degradation. Therefore, the relationship between load time and peak exothermic temperature was examined. Table 2 shows the relationship between load time and peak exothermic temperature under conditions of 80°C and 95%RH.

[0027] [Table 2]

[0028] The degradation rates in Table 2 are calculated based on the results in Table 1, assuming a minimum exothermic peak temperature of 121.3°C and a maximum of 136.1°C. To confirm the responsiveness of this degradation rate, a Weibull plot of load time and degradation rate was performed. The results are shown in Figure 2.

[0029] The results in Figure 2 confirm that load time and degradation rate show a high correlation in the Weibull plot. In other words, it can be concluded that load time and degradation rate follow a Weibull distribution. Furthermore, since the shape parameter m (slope of the regression equation) is m>1, it is clear that the assumed failure mode (failure rate increasing type such as wear failure) is present. Based on these results, it can be concluded that estimating degradation progression using the heat peak temperature has sufficient explanatory power.

[0030] To perform calculations using the Eyring model, it is necessary to determine the intrinsic activation energy and humidity acceleration coefficient. Both of these coefficients can be determined experimentally.

[0031] First, we derive the activation energy. Activation energy (Ea) is the energy barrier that exists during the process of transitioning from a normal state to a degraded state, and in temperature-accelerated testing, it is a parameter that represents the acceleration with respect to temperature. a E takes on different values ​​depending on the failure mechanism. a A larger value indicates greater temperature acceleration, while a smaller value indicates less temperature acceleration. a This is calculated using the Arrhenius equation. Equation 3 shows the Arrhenius equation for failure time L2.

[0032]

number

[0033] Taking the logarithm of both sides of equation 3 yields equation 4.

[0034]

number

[0035] Equation 4 shows that the logarithm of lifetime and the reciprocal of temperature form a straight line, and the activation energy is obtained by multiplying the slope of this line by Boltzmann's constant. In other words, by deriving a regression equation from experimental data of lifetime (failure time) under different temperature conditions, E a It becomes possible to calculate this.

[0036] In this embodiment, humidity is also one of the parameters, E a In the derivation experiments, it was necessary to fix an arbitrary value. Therefore, the humidity was set to 95%RH, and load experiments were conducted under three conditions: 50°C, 65°C, and 80°C. The results are shown in Figure 3 as a Weibull plot.

[0037] The regression equation calculated from the Weibull plot showed that the coefficient of determination R was the same for all conditions. 2 The result was 0.99 or higher, confirming that the estimated regression equation fit very well. Furthermore, all shape parameters were m>1, and the values ​​of the shape parameters were relatively close across all three conditions. Therefore, it can be concluded that the experimental results obtained in this study are causing the anticipated failure mode (wear failure).

[0038] Next, the activation energy was calculated using these results. The time to life (failure time) for each condition was calculated using the regression equations shown in Figure 3. The results are shown in Table 3.

[0039] [Table 3]

[0040] The results in Table 3 are shown in Figure 4 in the form of the logarithm of the lifetime and the reciprocal of the temperature (absolute temperature). From these results, the activation energy E of the sample resin can be determined. a This can be calculated using formula 5.

[0041]

number

[0042] As described above, the activation energy E of the sample resins targeted in this study a It is 0.665 (eV).

[0043] Next, we will derive the humidity acceleration coefficient. The humidity acceleration coefficient will be derived using Equation 1, which is the general formula for the Eyring model mentioned earlier.

[0044] First, take the logarithm of both sides of equation 6 to obtain equation 7.

[0045]

number

[0046]

number

[0047] In the right-hand side of equation 7, if we fix T and limit the parameters to relative humidity (RH) only, then everything except n ln(RH) becomes a constant. In other words, equation 7 shows that the logarithm of the lifetime and the logarithm of the relative humidity are linear, and the slope of this line is the humidity acceleration coefficient n.

[0048] To clarify the relationship between time to life and relative humidity, load experiments were conducted at a temperature of 80°C under three conditions: 65% RH, 80% RH, and 95% RH. The results are shown in Figure 5 as a Weibull plot.

[0049] The regression equation calculated from the Weibull plot showed that the coefficient of determination R was the same for all conditions. 2 The value was 0.98 or higher, confirming that the estimated regression equation fit very well. Furthermore, all shape parameters were m>1, and the values ​​of the shape parameters were relatively close across all three conditions. In other words, even under humidity fluctuation conditions, the experimental results obtained in this study indicate that the expected failure mode (wear failure) is occurring.

[0050] Next, the humidity acceleration coefficient was calculated using these results. The time to life (failure time) under each condition was calculated using the regression equations shown in Figure 5. The results are shown in Table 4.

[0051] [Table 4]

[0052] The results from Table 4 are shown in Figure 6 in the form of logarithmic values ​​of lifetime and logarithmic values ​​of relative humidity. The humidity acceleration coefficient n for the sample resin was calculated to be -5.261 from the slope of the regression equation.

[0053] From the above results, the activation energy E required to determine the acceleration coefficient A using the Eyring model in Equation 2 a The humidity acceleration coefficient n was determined.

[0054] To estimate the lifespan of a resin under standard conditions, it is necessary to measure its lifespan under accelerated conditions. As an example, accelerated conditions can be set to a temperature of 80°C (T A ), relative humidity 95% (RH A Set to ). Under these acceleration conditions, the time L until the resin reaches the end of its lifespan. A The following was calculated: As a result, time L under acceleration conditions A The duration was 22.72 hours.

[0055] As mentioned above, E a is 0.665, and n is -5.251. Under these conditions, several assumed standard conditions (temperature T) N , relative humidity RH N Time to life of resin in ) L N The results of the calculation are shown in Table 5.

[0056] [Table 5]

[0057] The above assumed standard conditions are just an example; the lifespan of the resin can be estimated using Equation 2 under any temperature and relative humidity conditions.

[0058] The resin life estimation method of this embodiment can also be used for resin selection. Multiple types of resins are prepared in advance, and the activation energy E for each resin is determined. a The humidity acceleration coefficient n is calculated. Here, multiple types of resins refer to different lots of the same resin, polyamide elastomers with different amounts of soft segments, those with different copolymer composition ratios, or the same resin with different contrast agents, etc., for the purpose of analysis. Next, the lifespan of each resin under accelerated conditions is measured, and the time until the resin reaches the end of its lifespan under standard conditions is estimated using Equation 2. Based on the results, the resin can be selected.

[0059] As described above, the (1) resin life estimation method according to this embodiment is a resin life estimation method that estimates the time it takes for a hydrolyzable resin to reach its lifespan at an arbitrary temperature T and relative humidity RH, and measures the relationship between the exothermic peak temperature and load time measured using a differential scanning calorimeter of the resin at a constant relative humidity RH1 at multiple temperatures T, calculates the time L until the resin reaches its lifespan at each temperature T, and extracts the activation energy E from the relationship between temperature T and time L. a The process involves calculating the following: measuring the relationship between the resin's heat-generating peak temperature at a constant temperature T1 and the load time at multiple relative humidity levels RH, calculating the time L until the resin reaches the end of its lifespan at each relative humidity level RH, and calculating the humidity acceleration coefficient n from the relationship between relative humidity level RH and time L; and temperature T A and relative humidity RH A Time L until the resin reaches the end of its lifespan under acceleration conditions A The process of calculating the Boltzmann constant, with temperature T being k. N and relative humidity RH N Time L until the resin reaches the end of its lifespan under standard conditions N The method comprises a step of calculating from equation 2. This resin lifetime estimation method can accurately estimate the lifetime of a resin under arbitrary temperature and relative humidity conditions because it can accurately calculate the activation energy and humidity acceleration coefficient by measuring the peak exothermic temperature.

[0060] (2) In the resin life estimation method described in (1) above, the resin may be a resin material used in medical devices. This allows for the appropriate setting of the expiration date of medical devices.

[0061] (3) The resin selection method according to the present invention involves preparing multiple types of resins, estimating the time until each resin reaches its lifespan using one of the resin lifespan estimation methods described above, and selecting a resin based on the time until the resin reaches its lifespan. The resin selection method configured in this way allows for the appropriate selection of a resin based on its expected lifespan.

[0062] It should be noted that the present invention is not limited to the embodiments described above, and various modifications can be made by those skilled in the art within the technical framework of the present invention.

Claims

1. A resin lifetime estimation method for estimating the time it takes for a hydrolyzable resin to reach its lifespan at an arbitrary temperature T and relative humidity RH, constant relative humidity RH １ The relationship between the peak exothermic temperature and load time, measured using a differential scanning calorimeter of the resin at multiple temperatures T, is measured, and the time L until the resin reaches the end of its lifespan at each temperature T is calculated. The activation energy E is then derived from the relationship between temperature T and time L. ａ The process of calculating, constant temperature T １ The process involves measuring the relationship between the peak heat generation temperature of the resin and the loading time at multiple relative humidity levels RH, calculating the time L until the resin reaches the end of its lifespan at each relative humidity level RH, and calculating a humidity acceleration coefficient n from the relationship between the relative humidity level RH and the time L. Temperature T Ａ and relative humidity RH Ａ The time L until the resin reaches the end of its lifespan under the acceleration conditions Ａ The process of calculating, Let the Boltzmann constant be k, and temperature T Ｎ and relative humidity RH Ｎ time L for the resin to reach end-of-life under the standard conditions of Ｎ a step of calculating by the following formula: a resin life estimation method comprising same.

2. The resin life estimation method according to claim 1, wherein the resin is a resin material used in medical devices.

3. Prepare several types of resin, The time until each resin reaches the end of its lifespan is estimated by the resin life estimation method described in claim 1 or 2. A method for selecting a resin, which involves selecting a resin based on the time it takes for the aforementioned resin to reach the end of its lifespan.

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