Ken Mintz Composite Information pg.4 ==
Page 1 Introduction=================================
Page 2 A Short Description of Fiber Types and Properties=====
Page 3 Advantages/Disadvantages of Composites===========
Page 5 Setting Up The Shop===========================
Page 6 Preparing to Layup and More Tools=.==============
Page 7 The Core and Its Preparation.====================
Page 8 Hot Box Pictures and Dimensions.======.==========
163 Home Page====================================
Subject: Composites Corner No. 4 - The Matrix
This time I will discuss the matrix (not the movie). The matrix is the material in which the fibers of a composite
structure are embeded. It has no particular orientation as is necessary for the fibers to function properly and
its main function, in fact, is to hold these same fibers in the proper orientation in order to permit them to bear
the loads that they are intended to carry. In performing this function it gives the composite part its shape.
It acts as a glue and distributes loads across the fibers within it. To some degree it protects the fibers from
environmental effects such as moisture, chemicals, impacts and abrasion. It will protect the fibers from UV
attack but often at the expense of the matrix material itself. All of the composite matrix materials that are
currently in use by the experimental aircraft set require extensive protection against UV assault. This means
paint. (Remember those friends that I mentioned earlier in this series)?
The structural properties that the matrix contributes to the total composite component are compression
resistance, shear strength, delamination reistance and its temperature limitations (service temperature). These
properties are dominated in the finished composite part by the matrix material with some minor contributions
from the fibers in these areas. The main function of the fiber materials are to bear loads in tension.
There is a rather wide variety of matrix materials in use in the composties industry to include organic, thermoplastic,
carbon (carbon-carbon structures), ceramic and metal matrix types. The one that we homebuilders use almost
entirely is the thermoset class of matrix materials. This class includes the liquid resins - polyesters, vinyl esters
and the good old epoxys. It also includes the phenolic type. Phenolic is perhaps the oldest of the modern
composite matrix materials.
The three liquid resins all use additive materials often generically called catalysts or hardeners to cure the
resin type used. They are not the same and function quite differently from each other chemically.
Both of the "esters" use what are commonly called a catalysts. (The chemists tell me that these are not really
catalysts but the name stuck). They allow the "ester" resins to form long molecular chains called polymers
in the process called polymerization. They allow this to happen because the "catalysts" remove inhibitors
added during manufacture that prevent polymerization that would automatically occur without them.
The most common catalytic materials used with polyester and vinyl ester resins are Methyl Ethly Ketone Peroxide
(MEKP) and Benzol Peroxide (BPO). The rate of cure of these resins is regulated by other additives called
promoters and accelerators. Cobalt Napthenate (CoNap) is a common promoter and Dimethyl Aniline (DMA)
is a common accelerator. Styrene is used as a dilution agent in these resins and is the source of the smell.
(Don't worry, there's no test. Just don't get the idea to mix up these materials. Some of them will blow up and
catch fire). The rate of cure can be controlled within limits by adding extra catalyst ( "hot mix" ) and visa-versa.
High outside temperatures will also accelerate the curing process.
Epoxies use a hardener and not a catalyst for their curing process. The hardener represents a much larger
percentage by weight of the epoxy matrix material than is the case with the "esters" where very little of the
catalyst is used. The hardener can be from 10% to 50% of the weight of the matrix material in proportion with
the base resin. The epoxy hardeners do their curing by "cross linking" their polymers with the base resin
polymers. This makes accurate weighing and thorough mixing critical to achiving a good epoxy cure. Adding
or reducing the ratio of hardener to base resin will not speed up or slow down the cure rate in an epoxy and
can severely alter the strength of the matrix. Cure rate is achieved only by either changing the type of hardener
used or by varying the temperature. Good temperature control is necessary in your construction space.
Note that epoxy is a class of resin in itself and is not synonymous with the polyesters and the vinyl esters.This
bears on their compatibility with each other as far as mixing structures made of the different resin types. For
example, repairing a structure made of one type of resin using the other will not achieve a very strong bond
between the two especially between the epoxies and the "esters". Likewise grafting one part onto another of a
different resin type will be weak as well. The bond that is achieved is mostly a mechanical one and is insufficient
for structural purposes but can appear stronger than it really is. In short, when grafting new structure to old or in
making repairs be sure to use the same type of resin in the new part or repair that was used in the old. You can
make discrete parts from different resins so long as they do not need to be bonded to each other chemically by
the curing processes described. For instance, you can make your wheel pants out of a different resin (or fiber)
than that of you aircraft as long as it is attached by bolts, screws or other mechanical means and meets the
Information about the compatibility of resins can be obtained usually from the plane's designer or kit manufacture
as well as the maker of the resins. Keeping good records on this subject during the building or your aircraft will
be of much help in the future.
I will try to address repairs and modifications in later issues of Composites Corner..
Phone: (702) 567-1938