Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-11-30
2004-01-06
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C073S054410, C073S573000
Reexamination Certificate
active
06675112
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the monitoring of solidification of plastics resins, and in particular relates to monitoring the curing of adhesively bonded or sealed joints, monitoring the cure of thermosetting resins and monitoring the cure of composite materials comprising plastics resins.
As a thermosetting polymer cures, its temperature generally rises and the viscosity falls until the gel point is reached, after which the viscosity rises rapidly until solidification is complete, whether this is in a composite structure or an adhesive bond, the way in which components are handled during this time is critical. For example in the case of an adhesive bond, if the components are brought together under pressure too early, the low viscosity resin will be forced from the bond, producing less than optimum bond thickness and large amounts of spew. If the components are forced together too late the adhesive will have begun to gel and a network will have begun to form, thus the molecular structure of the adhesive will be disturbed.
Similar situations exist in composite manufacture e.g. pressure moulding of composite panels in stop-less or picture frame moulds. In both types of application, once the solid polymer structure begins to form the components must be held exactly in place. Once sufficient strength has built up (the ‘green’ strength has been reached) the completed structure may be removed from the mould or jig and curing completed in a second oven. In order to optimise production time and quality it is important to know when
i) the gel point is reached,
ii) green strength has developed, and
iii) cure is complete.
Hitherto, methods of determining state of cure of plastics resins have been limited to destructive examination and testing or off-line techniques. Ultrasonics, Radiography and low frequency techniques are known in the non-destructive testing (NDT) of adhesive bonds and composite materials. However current low frequency techniques are limited to the detection of disbonds etc., in a polymer after curing. The usefulness of radiography is severely limited by the limited range of adherend, which may be used, and by the orientation of the defects being sought.
Ultrasonic techniques are limited by both the type of adherend and adhesive. In particular, if either adherend or adhesive is significantly acoustically absorbing, then it is not possible to apply such techniques.
U.S. Pat. No. 4,862,484 is concerned with the measurement of the dynamic viscosity of a viscous medium using an acoustic transducer in the temperature and pressure environment of the medium and spaced therefrom. The specification discloses the use of an ultrasonic frequency and the viscosity of the fluid is calculated using the shift between the first and second resonant frequencies and the difference between first and second band widths. Such a system is limited, effectively, to measuring frequency in a substantially liquid material and does not permit the monitoring of the cure of a material after effective solidification.
U.S. Pat. No. 4,758,803 relates to the detection of ultrasonic properties of fibre reinforced plastics during the curing process. Such a system has only limited application since ultrasound is absorbed by some polymers and reinforcers and is not effective in the monitoring of post solidification curing.
U.S. Pat. No. 4,455,268 is concerned with an automatic system for controlling the curing process of structures formed of fibre reinforced composite materials in an autoclave. The specification is concerned with the measurement of viscosity data through measurements of attenuation of ultrasonic waves in the composite material of the structure. This specification teaches that attenuation is directly related to the viscosity of the structure which is then compared with a “model” profile.
None of these techniques are able to detect poor cohesion or adhesion in-situ and do not permit effective continued monitoring after solidification.
The foregoing techniques may be used in the non-destructive testing (NDT). of composite materials, however significant problems exist. The use of radiography is limited because composite materials, particularly carbon fibre reinforced composites, are weak absorbers of. X-rays. Thus it is difficult to produce high contrast images. Ultrasonic techniques can be used but problems can occur due to scattering produced from filler particles and fibre reinforcement.
Methods currently used to monitor cure include quantitative. (wet) analysis, Fourier transform infrared, Raman, near infrared, nuclear magnetic and electron spin resonance spectroscopy, DSC, DPC, torsional braid analysis, dynamic mechanical thermal analysis, dielectric techniques, Theological methods and ultrasonic techniques.
Quantitative analysis. requires the removal of a sample of the resin to determine the content of an active group involved in the cross-linking reaction, for example epoxy or hydroxyl in the case of epoxy resins. Although the technique may be used to detect off-stoichiometric effects in a cured resin it is not feasible to use it as a cure monitoring technique. Spectroscope techniques provide information regarding the state of the chemical reaction and are useful as research tools. However many such techniques require access to the surface of the curing polymer which is clearly difficult in many industrial applications e.g. large composite structures and adhesive bonds.
This problem can be overcome to some extent by using optical fibres, however these must then remain in the cured structure and may potentially act as crack initiators. This is obviously undesirable, particularly in critical structural applications. Spectroscopic techniques require the polymer to be placed in a particular environment e.g. Nuclear Magnetic Resonance where the sample is placed in a strong fixed magnetic field. Again, this is not feasible in many industrial applications.
Dynamic mechanical thermal analysis, DSC and DPC are again useful research laboratory tools. However the techniques also rely upon removing a sample of curing polymer and placing it in a controlled environment within the instrument, thus in-situ measurements are not possible. Dielectric techniques measure changes in permittivity that occur during the polymerisation process as a result of changes in the concentration of functional groups. Such methods involve placing electrodes in the curing polymer, these remain in the polymer once it has cured and may act as crack initiators.
Torsional braid analysis involves the impregnation of a glass fibre braid with uncured liquid monomer. This braid is then subjected to a sinusoidal torsional force and complex modulus determined. Again this involves leaving a foreign body in the cured resin and whilst this may be acceptable in composite manufacture where crack propagation may not be critical, it would not be acceptable in general adhesive bonding.
A number of Theological techniques are available which essentially follow the physical properties of the resin until the point when it becomes solid. These are laboratory techniques and not suited to making in-situ measurements.
Finally, ultrasonic techniques do potentially allow the NDT/NDE of a curing polymer in-situ. However, like the ultrasonic flaw detection techniques, they are not suited to adherend/adhesive combinations that are highly acoustically absorbing.
Accordingly there is a need for a non-destructive method for the monitoring of the progress of curing in plastics resin materials or structures comprising those materials.
There are a number of low frequency tests already in the market place which are used to detect dis-bonds and delaminations in formed composites. Many of these are based on the ‘coin tap’ test. In its simplest form the ‘coin tap’ test involves tapping the component under test with a metallic striker. Defective areas will sound ‘dead’ due to absorption at the defect site. For example, tapping a good railway wheel will produce a clear sustained ring, whilst a cracked wheel produces a short ‘dead’ ring. Esse
Nixon & Vanderhye PC
Shah Kamini
The Manchester Metropolitan University
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