LF Technology - unique epoxy foil

What is LF Technology?

The LETOXIT® Foil Technology (LF Technology) is our unique patented technology, which allows dry way of laminate production based on the impregnation of reinforcement with LETOXIT® foil resin at increased temperature and pressure.

Prepregs are materials used for years, but they are very expensive and many prepreg types are necessary, if composite structure should be optimised well. RTM is a relatively cheap method, big parts can be manufactured. The main problem is impregnation of the reinforcement. Therefore only special types of the reinforcement have to be used and the result is relatively low reinforcement content in a final composite part. In recent years, technology called RFI – Resin Film infusion appeared. The production cycle is performed usually in the autoclave under increased temperature and pressure, where the fibre infiltration and the composite consolidation occur in a single step process. Typically, the RFI element consists of a thermoset resin film placed between one side of a metal tool and a dry textile fibre preform. Problems with conventional RFI appear if sandwich structures are manufactured; honeycombs are filled with resin and foam prevents from impregnation of the sandwich skin in opposite site of the resin layer.

LF Technology SCHEME

Concerning these disadvantages of conventional processes and a need of universal low cost technology, 5M s.r.o. developed and patented a very efficient technique of production of sophisticated and relatively cheap composite structures, which is called Letoxit Foil Technology (LF Technology).

LF TECHNOLOGY is epoxy resin in the form of flexible foil, which softens at increased temperature and it is able to impregnate originally dry reinforcement and cure in one step.

During manipulation, production and curing doesnt release any hazard vapours and foil is very friendly to use.

LF Technology allows the application of various types and numbers of layers at different places of the construction.

The products made by the LF Technology are comparable with products made by the prepreg technology. In comparison with prepregs has LF Technology big advantage in variability, i.e. in production can be used dry fabric as required (the same like in usual hand lay-up) and that saves money, because there is no need to store huge amount of prepregs in freezer.

Advantages:

substite for prepregs with better logistics conditions - reinforcement you can choose, in freezer store only resin
variability of reinforcement
dry way of lamination
3x faster production compare with liquid resins
impregnation and curing in one step
great ratio resin/reinforcement (40:60)
very nice and smooth surface
no hazard vapours
very friendly to use
accurate defined composite structure in everyposition in the final product

Practical information about application and working with LF Technology you can find in this article, which was issued in magazine Czech aerospace processing (covered by Aeronatical research and test institute)

Technical data

Possible reinforcement	glass, carbon, aramid, basalt or hybrids. rovings, mats, unidirectionally oriented reinforcement or 3D fabric
Area weight	120 g/m² - 840 g/m²
Curing temperature	120°C for 60 minutes 80°C for 240 minutes
Required vaccum	0,075 - 0,09 MPa
Roll width	250 mm 1000 mm
Possible core material	honeycombs made from Nomex, aluminium, paper, PP, etc. foams made from PUR, PS

Whole product range covers many types with different characteristics, for example there are types with high temperature resistance (250°C), or fulfils FAR 25 and FAR 23 standards, or type developed specially for lamination of carbon fabrics.

LETOXIT^®Foil

TYPE	RESIN	CURING TEMP.	CURING TIME	Max. Tg	DESCRIPTION
LFX 023	epoxy	120°C	60 min.	100°C	basic type
LFX 035	epoxy	120°C	60 min.	125°C	low viscosity, for carbon fabrics
LFX 038	halogenated epoxy	120°C	60 min.	135°C	self extinguishing (FAR 23)
LFX 040	modificated kyanoester	120°C	60 min.	250°C	high Tg
LFX 044	modificated epoxy	120°C	60 min.	105°C	high impact strength
LFX 054	epoxy	120°C	60 min.	155°C	higher Tg, low viscosity
LFX 055	epoxy	120°C	60 min.	155°C	higher Tg, low stickiness
LFX 056	epoxy	120°C	60 min.	125°C	self extinguishing (FAR 25)
LFX 060	epoxy	80°C	240 min.	125°C	curing at 80°C, low viscosity (for carbon)
NEW! LFX 062	phenolic	120°C	45 min.	103°C	self extinguishing (DIN 5510-2)
LFX 162	epoxy	80°C	240 min.	100°C	low curing temp., higher stickiness

LF Technology - technical data

Application

LF Technology is suitable for any composite structure, where balanced weight and mechanical properties ratio are required. Typical applications are tin shell structures with high reinforcement volume, locally added reinforcements and/or with different required local properties (for example reinforcement orientation) because these structures cannot be manufactured by conventional technologies without highadded cost of material and work. The conventional composite structures, originally made by RTM, VARTM, Hand-lay up, are possible, of course. but due to lower requirements on mechanical properties on parts made by these conventional technologies, economy of LF technology (RFI) must be taken into account.
Potential applications are:

Automotive – car bodies, covers, hoods
Transportation – hoods, side panels, doors, racks
Aerospace – covers, doors and all types of panels
Building – tables, eye-catchers
Medical – orthosis and prosthesis

Sample 1:

First application was an engine hood of ultralight aeroplane prototype CHS 701SP (producer Czech Aircraft Works), originally composite part made by hand-lay up from unsaturated polyester resin. 40% weight decrease was achieved using LF Technology This weight decrease appears and the same stiffness remains due to use of local reinforcement with Nomex honeycombs in the top of the hood. Foam was applied in the upper part. Other important benefit was higher thermal stability. Good surface of the part was achieved although gelcoat was not applied.