Impregnation of Textile Fibers, and Fiber Reinforcements of Resin-Matrix Composites

W.M. Zadorsky

1. Introduction

Resin-matrix composites and especially fiber-reinforced plastics (FRPs) have a wide variety of applications ranging from mass-produced tennis rackets to automobiles to complex aerospace structures. Their reinforcements may be in the form of cloth, mats, strand and other fibers of glass, carbon/graphite, aramides etc.

FRP quality depends heavily on the bonding strength between its polymer matrix and the reinforcement. Impregnation may be a decisive step in achieving good adhesion and strong bonding. During impregnation, the entire surface of the reinforcement must come into contact with the matrix material. Otherwise, gas-filled bubbles, crevices and other discontinuities may be present that will adversely affect the properties of prepreg, wet lay, and finished material or components.

Among the fiber material filaments, however, there always are small pores and interstices filled with air. The air may block the impregnant penetration into some of such capillary type passages. No wetting of their surfaces will therefore occur, resulting in poor bonding.

Vacuum, high pressure and other techniques are conventionally used for a more nearly complete penetration of the impregnant. This, however, is seldom achieved, even though expensive equipment, long processing times and high additional costs may be involved.

The project is aimed at developing a quick, effective and low-cost method to ensure perfect impregnation of fiber reinforcements with organic impregnants.

2. Project Description
2.1. Process Development

The method eliminates vacuum or high pressure equipment and may require only minor, if any, modifications in the existing equipment. It relies on a simple three-step treatment of the fiber material directly before impregnation. The pretreatment removes all air trapped in the open pores and involves the following steps carried out in quick succession:

heating the fiber material,
introducing a specific nonreactive gas, and
desorption of the gas.

It activates every open pore/interstice and results in their quick and complete filling during the impregnating step.

2.2. Materials and Equipment

The nonreactive gas characteristics and the timing are unique to each fiber/resin system. This necessitates their tailoring to the system at hand. The gas will invariably be selected among those inexpensive and readily available ones.

The equipment will be the same for any system to be processed. The process allows continuous and batch operation alike.

2.3. Process and Product Characteristics

Experiments with graphite and glass reinforcements revealed that the method may improve the fiber surface area coverage by 30 % and shorten the impregnating step time 2-fold. Importantly, no selective sorption of any components of impregnant occurred.

These improvements were even greater with high-viscous impregnants.

The laboratory test data were validated by reliable service of prepregged components in some high-performance applications.

3. Novelty

The method is believed to be patentable because it has not been disclosed and no analog to it has been found in the literature.

4. Marketing

The market for resin-matrix composites, particularly FRPs, is ever expanding in commodity industries and high-performance applications alike.

The cost of equipment adaptation to the new process is negligible as is the running cost for the new appliance. It is further assumed that the existing vacuum and/or high pressure equipment used for impregnation may be taken out of service. The economic gain from the new method may thus be roughly estimated from the following relationship [1]:

Processing cost, $/unit product
= (Hourly machine cost, $/h)/(Production rate, unit/h)
= (Initial hourly cost - Vacuum/pressure equipment hourly cost, $/h)/2,

which gives at least a 2-fold reduction in the impregnation cost. The product cost is further cut down due to an anticipated 30% drop in the rejection rate.

At the customer's side, the gain will come directly from reduced price and also indirectly from improved service properties and durability of product.

5. Application

Aerospace and automotive components, building materials, coated fabrics, construction components, conveyor belts, machine components, protective clothing, ships, sporting goods, and tires are among the first-priority applications.

The method can also be readily adapted for the textile industry at large. Here, all fibers and fabrics undergo some kind of "impregnation" via e.g. dipping for purposes of dyeing, discharging, proofing etc. The method offers increased production rates and improved product quality.

References

1. Engineering Plastics. Engineered Materials Handbook, vol. 2. ASM International, Metals Park, OH, 1988, p. 648.

Contact:
Prof.William Zadorsky
PEF&PCPC

Information supplied by the Author October 1999.  Page last updated: July 03, 2005