Laminates resin content

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. 

Properties measured using 1—1.5 mm laminates glass fabric U.S. 2116, resin content 45% by weight. 

Figures 23.21 and 23.22 show how two variables, moulding pressure and resin content, affect the mechanical properties of a laminate.

Figure 23.22. Effect of resin content on strength properties of a high-density Kraft paper laminate.

Because of the lack of solubility in the usual solvents, aniline-formaldehyde laminates are made by a pre-mix method. In this process the aniline hydrochloride-formaldehyde product is run into a bath of paper pulp rather than of caustic soda. Soda is then added to precipitate the resin on to the paper fibres. The pulp is then passed through a paper-making machine to give a paper with a 50% resin content. 

The properties of the laminate are dependent on the resin and type of glass cloth used, the method of arranging the plies, the resin content and the curing schedule. Figure 29.4 shows how the flexural strength may be affected by the nature of the resin and by the resin content. 

Figure 29.4. Influence of resin content on the flexural strength of glass-cloth laminates made with two silicone resins A and B. (After Gale " )...

In overcoming the shortcomings of the earlier models, Dave et al. [21,22] proposed a comprehensive three-dimensional consolidation and resin flow model that can be used to predict the following parameters during cure (1) the resin pressure and velocity profiles inside the composite as a function of position and time, (2) the consolidation profile of the laminate as a function of position and time, and (3) resin content profile as a function of position and time. 

A generalized three-dimensional resin flow model has been developed that employs soil mechanics consolidation theory to predict profiles of resin pressure, resin flow velocity, laminate consolidation, and resin content in a curing laminate. 

Resin content results confirmed the resin pressure results, showing a large resin content gradient (Fig. 10.7) existed at the edge of the laminate. The figure also illustrates that resin bled further into the bleeder when a large gap was used between the laminate edges and the dams. 

Laminate manufacture involves impregnation of glass cloths with a liquid resin in a dip coating operation. The treated glass cloths are dried in an oven and the resin pre-cured. Even if most of the products are applied as aqueous solutions of the prepolymers, powdered materials are also available. The advantage of the powdered products is their better chemical stability compared to the aqueous solutions. The resin content of the final products lies between 30 and 70%. 

The treated veneer should then be close piled for one to two days, with a water proof cover over it, to allow for equalization of the resin content by diffusion. The veneer can then be dried and the resin polymerized in a continuous veneer drier or in a dry kiln. Real fast initial drying should be avoided to prevent excessive migration of the as yet uncured resin to the surfaces. The treated veneer is then laminated into panels of any desired thickness in a hot press using phenolic glue (45). 

DMA analysis substantiated the possible morphological features that could partially account for the trend in mechanical and fracture properties of the glass laminate composites [146]. The DMA, done at a frequency of 1 Hz, are given in Fig. 12, where the percentage retention of the room temperature storage modulus (E ) is plotted as a function of temperature to normalize the anomalies resulting from differences in resin content for different systems. EPOBAN-modified... 

Several materials combinations were examined and it was determined that fabrics made from fibers with expansions significantly lower than that of conventional E-glass have to be used to make boards that have the desired thermal expansion characteristics. Aramid reinforced laminates appear to be particularly useful in this application and their thermal expansion characteristics were studied using several experimental techniques and using a computer model to study the effect of resin content, resin modulus and resin Tg on these materials. 

The model and the data show that the tnermal expansion of a fiber reinforced laminate is controlled by the following factors Tg of resin (if it is within range in which thermal expansion is measured), volume fraction of fiber and matrix, moduli of fiber and matrix as a function of temperature, and expansion coefficients of fiber and matrix as a function of temperature. For example, the model predicts that for Kevlar reinforced laminates made with the epoxy formulation discussed here, a 60% by weight resin content laminate would have a TCE of 7.5 x 10 /°C and for a 35% by weight resin content the TCE would be 4.3 x 10- /°C. 

Data obtained from the model and experimentally obtained data using the horizontal dilatometer are presented in Figure 7 for laminates with 60% by weight resin content and Figure 8 for laminates with 35% by weight resin content. Experimental and predicted data is once again in close agreement. 

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