Laminates Cure, degree

Cure of the resin plays an important part in the chemical resistance of the thermoset. Improper curing will result in a loss of corrosion resistance properties. Construction of the laminate and the t)q)e of reinforcing used also affect the corrosion resistance of the laminate. The degree and nature of the bond between the resin and the reinforcement also plays an important role. 

Laminates. Laminate manufacture involves the impregnation of a web with a Hquid phenoHc resin in a dip-coating operation. Solvent type, resin concentration, and viscosity determine the degree of fiber penetration. The treated web is dried in an oven and the resin cures, sometimes to the B-stage (semicured). Final resin content is between 30 and 70%. The dry sheet is cut and stacked, ready for lamination. In the curing step, multilayers of laminate are stacked or laid up in a press and cured at 150—175°C for several hours. The resins are generally low molecular weight resoles, which have been neutralized with the salt removed. Common carrier solvents for the varnish include acetone, alcohol, and toluene. Alkylated phenols such as cresols improve flexibiUty and moisture resistance in the fused products. 

Barcol hardness Also called Barcol impresses It is a measure of the hardness of a plastic, that includes laminate or reinforced plastic, using a Barber Coleman spring loaded indenter. Gives a direct reading on a 0 to 100 scale higher number indicates greater hardness. This test is often used to measure the degree of cure for plastics, particularly TS plastics. 

Tool-mounted optrode (TMO). The cure-monitoring experiments described here were conducted with a "tool-mounted" optrode (TMO) arrangement (5,6) (Figure 2) which is Ideally suited for the manufacturing environment where minimum interference with the laminate layup work is desirable. The use of a tool-mounted optrode is as simple as the use of tool-mounted thermocouples currently in wide use. Indeed, the TMO provides viscosity/degree-of-cure information on the cure state of the surface layer only. However, knowledge of the cure state of the surface layer permits determination of the cure states in the bulk based on the available models (1,2). 

As the thickness of the laminate increases, the strength of this thermal spike and the degree of thermal lag during heat-up increases. Figure 8.8 shows the results for a 62.5-mm (500 ply) laminate of the same material. Now the center-line temperature never reaches the autoclave temperature during the first dwell, and the thermal spike during the second dwell is nearly 135°C. The thermal spike is directly related to the release of internal heat during cure. The thermal lag is a manifestation of the low thermal diffusivity of polymer matrix composites. 

In Equation 8.23 the superscripts refer to the 0 and 90-degree plies and the subscripts 1 and 2 refer to the axis directions as depicted in Figure 8.10. Note that the laminate stiffnesses Qy in Equation 8.23 are cure dependent [i.e. Qy(a)]. 

If the elapsed time between lamination and food contact is too short, imreacted monomeric isocyanate can be available to react with water to form low molecular weight amines. The presence of aromatic amines can be an issue with the use of MDl or toluene diisocyanate (TDl) containing isocyanate systems. Often aromatic isocyanates are selected to obtain good adhesion under moist and low temperature conditions. Aromatic isocyanates are also cheaper than aliphatic systems. The crystallinity of the film can reduce permeation rates of monomeric isocyanates into adjacent film layers. Other factors that can influence the rate/degree of cure (isocyanate reaction) include the following ... 

Plastics - differential scanning calorimetry - general principles Determination of characteristic reaction cure temperatures and times, enthalpy of reaction and degree of conversion Glass-transition temperature and cure factor for laminates and printed circuit hoards Replaced hy ASTM D3418... 

There are different grades of each of these materials according to the overall molecular weight and the degree of substitution. These polymers are used as components of systems with unique adhesive properties for example, in the manufacture of safety glass laminates (poly(vinyl butyral) and mixed derivatives) and of metal-to-metal adhesive (poly(vinyl formal) cured with phenolics and other resins). Reactions of poly(vinyl alcohol) with acids or anhydrides occur as normal esterifications, a route used to synthesize polymers and copolymers that cannot be readily formed by conventional polymerization (e.g., when the reactivity ratios of the monomers are not suitable). 

Phenolic resins have been developed to allow their use as laminating resins in a water solution. After impregnation, cure is achieved by the addition of an acid catalyst, resulting in an exothermic reaction, and with emission of water vapour, which tends to cause an inherent high void content in composites. This problem, combined with a degree of chemical affinity for moisture, can lead to a level of water absorption up to three times greater than with an isophthalic polyester resin. 

In certain cases no surface treatment prior to joining is required. This is the case when the parent laminate is in the state where the formation of chemical bonds across the bondline is still possible. This requires a low degree of cure in the laminate and the time limit for this has to be defined separately for each resin and application. 

The resistance of a laminate to colour change on exposure is strongly dependent on the degree of cure before leaving the workshop. Exposure of adverse weather conditions, cold or moisture, before it is thoroughly cured will have an adverse effect on both the laminate strength and its weather resistance. 

Figure 24 Dimensionless temperature (a), degree of reaction (b), and viscosity (c), vs, processing time, compute at the sl and at the center of a typical epoxy matrix/carbon fiber laminate of 7mm half-thickness, cured isotermally at 177 C. (After Kenny et al., ref. 2).

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