Room temperature transformation - A175

Room temperature transformation
Document Type	Article
Document Identifier	175
Themes	Thermal management
Relevant Class	Equipment
Tags	Convective heating Equipment Thermal transformation Vacuum application Debulking Consolidation Object
Factory Cells	Thermal transformation
Prerequisites	Thermal transformation Heat transfer Convection Effect of equipment in a thermal management system

Introduction

Room temperature transformation is considered a piece of equipment from a thermal management standpoint. In this case, the room itself becomes the equipment as it is the room that controls the environmental conditions of the system. In that sense, a room, for the purpose of polymerization, effectively acts as a heating system for thermal transformation. The processing temperature is just much lower than other heating systems, such as ovens or autoclaves. The application of heat to the part functions according to the principles of convective heat transfer. Typically, this implies natural convection unless a fan (or other means) is used to forcibly blow air over the part. While the name implies room temperature (i.e ~20°C), in reality room temperature is used to denote any thermal transformation process taking place in ambient air. Therefore, the environmental conditions may fluctuate if no control systems are in place. The best example is curing outdoors. Here the part may be subject to drastic changes in temperature, airflow, UV radiation, and moisture depending on the weather.

Scope

The intent of this page is for the user to understand how a room may behave as a piece of equipment for thermal transformation. The page provides general information on room temperature curing. It does not delve into the scientific principles governing room temperature cure materials or the interaction between components in a room temperature setup.

Significance

Room temperature transformation is a low cost, easy-to-implement, and environmentally-friendly way to achieve low temperature thermal transformation of polymers, particularly thermosets. It is common to see room temperature heating setups across a wide range of industries. In fact, room temperature cure of polymers is commonplace in many households with the use of epoxies and other resins for repair operations and art projects. Generally speaking, room temperature heating is reserved for parts where low cost, rather than high-performance is the target.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Thermal transformation
• Heat transfer
• Convection
• Effect of equipment in a thermal management system - see convection section

Overview

Room temperature transformation refers to any thermal transformation process occuring at ambient temperature. That is, the temperature of the room in which the part is sitting. Curing of thermoset materials is the most common application of a room temperature transformation. Heat transfer to and from the part occurs via convection with the air. If the part is simply placed in a room and allowed to cure, the part is subject to natural convection of stagnant air. In such a scenario, the heat transfer coefficient (HTC) is usually relatively low, on the order of 2-10 W/m²K, with extreme cases up to 25 W/m²K. However, if a fan, or other means of forcibly moving air, is used, then the part is subject to forced convection and the HTC is increased. For example, if the part is cured outdoors with wind speeds ranging from just 1 to 4 m/s, the HTC may be in the range of 15-30 W/m²K. These values do depend on temperature, part shape, air density, and consistency of wind speed, however. In general, the greater the wind speed, the higher the HTC.

Vacuum may be applied to the part in a room temperature setup by means of an external vacuum pump. This allows for a pressure application on the part equal to atmospheric pressure, 101 kPa or 14.7 psi at sea level. At higher elevations atmospheric pressure drops and therefore so does the maximum potential pressure that can be applied to the part. It is also possible to increase the pressure by placing the part under a heavy object.

One benefit to room temperature setups, is that any free space can act as the equipment. Therefore, large individual parts or multiple parts may be placed in a given room and allowed to cure simultaneously. This is particularly true for curing outdoors where there is ample space to setup large parts for curing. Minor investments such as a table or other units for raising the parts off the ground are useful in allowing operators to easily move between parts and to reduce moisture ingress, oxidation of the tool, and other environmental impacts from a potentially damp ground in an outdoor setup.

Properties

Heating	Temperature	Pressure	Vacuum
Convective heating - typically natural unless a fan is used or the part is placed outside. HTC is usually 2-10 W/m2K for stagnant air. As air velocity increases, the HTC can be upwards of 15 W/m2K.	Typical temperature range: 20-25°C if placed indoors in a controlled environment. Outdoor temperatures are dependent on weather.	No pressure control. P\(_{app}\) = 0.	External vacuum control possible. P\(_{vac}\) = -101 kPa (-14.7 psi) at sea level.

Advantages

• Cheap; minimal equipment purchases, low cost tooling (no need for durable tools capable of handling multiple high temperature/pressure cycles)
• Easy to setup
• No energy input required; significant reduction in environmental impact and energy costs.
• Large amount of space for curing large parts or multiple parts - especially if cured outdoors
• Low HTC values if cured in stagnant air conditions; 2-10 W/m²K. Unlike for high temperature processing, where a high HTC is desirable, at low ambient air temperatures a low HTC may actually be advantageous (see the following page).

Limitations

• Not suitable for high temperature cure materials, such as aerospace-grade carbon epoxies
• Not suitable for melting and crystallization of thermoplastics
• No external pressure application; limited to vacuum pressure
• Generally reduced part quality; low fibre volume fraction, high porosity.
• Highly inconsistent HTC values compared with an oven, if cured outdoors.
• High variability between parts due to fluctuations of environmental conditions, especially in outdoor environments. Part quality is weather dependent.

Use

Room temperature transformation is ideal for thermoset materials that don't require high temperatures in order to begin their polymerization reaction. This includes families of resins such as polyesters and vinyl esters. The heat released from the polymerization reaction drives the curing process. This is in contrast to high temperature cure materials such as aerospace-grade carbon epoxies, where the heat released during polymerization may aid in bringing the part to temperature, but first significant heat must be input into the system and then maintained throughout the cycle.

The environmental conditions of the room (or space) in which the part is placed are arguably the most important parameters to consider for a room temperature setup. Ambient air temperature and the HTC have a dramatic affect on the thermal response of room temperature cure materials, and thus play a significant role in influencing their cure reaction. A part cured in a room indoors is likely to be exposed to temperatures on the order of 20-25°C. This may vary, however, if the door to the room is opened frequently, thereby introducing potential changes in temperature. This is especially important if any doors lead to outside the building, where the temperature may be drastically different than indoors. Regarding the HTC, unless a fan or other device is used to blow air over the part, the HTC is likely that of stagnant air. Opening and closing doors may also bring bursts of air currents, thus momentarily altering the HTC over the part. Unlike for high temperature processing, a lower HTC for a room temperature cure may actually be advantageous as the part is effectively insulated allowing the exothermic heat from the part's polymerization reaction to continuing advancing the degree of cure (DOC).

Curing a part outdoors brings even higher variability to the environmental conditions. Now the part is subject to seasonal and daily weather patterns. The ambient air temperature may fluctuate significantly depending on the time of year and the geographical location. This may mean that parts can only be cured in the summer when temperatures are high, or thermal management strategies must implemented for cooler seasons. Similarly, companies with multiple factories may choose to cure all their parts at one location where the local climate is relatively moderate and predictable. Even with such measures, air temperature may change notably from day-to-day or even within the same day. Temperatures at night are generally cooler than during the day, meaning parts cured overnight may reach a lower degree of cure (DOC) than parts cured during the day. In addition, wind will influence the HTC over the part. On days when it is calm, the HTC will be low, on the order of stagnant air. However, if the day is windy, then the HTC will increase drastically. Unlike air temperature, which may remain relatively stable over a season, wind speed can change drastically within a day.

To learn more about how ambient air temperature and the HTC influence the thermal response of room temperature cure setups, visit the following page.

Moisture uptake and UV radiation are other factors to consider for outdoor and, to a lesser extent, indoor curing. Covering the parts with an opaque cover can help mitigate UV degradation. Moisture absorption into the part is more complicated however. If indoors, an air conditioning system can be used to control the humidity. However, for outdoor cures, humidity will be determined by the relative humidity of the air. A possible strategy is to cure the parts in dryer climates, where relative humidity is low. However, dryer climates may also present larger temperature swings, especially between day and night.

Solidification of liquid thermoplastics may also occur at room temperature. A common example is the use of white glue or carpenter's glue, polyvinyl acetate (PVA). Here, exposure to air allows for water in the liquid glue emulsion to evaporate, thus solidifying the material.

Applications

Room temperature transformation is fairly common and may be found used for a wide variety of applications including:

• Marine - boat hulls
• Energy infrastructure - wind turbine blades
• Construction and repair - glues and resins for adhesive bonding and filling cracks
• Art - glues and resins for adhesive bonding or filling cracks
• Consumer goods

References

About	Help

A central processing theme in the manufacturing cycle. This theme is concerned with managing the thermal response of materials during storage and handling or parts/tools when they are subsequently heated.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.

A joining process where a polymeric material (the adhesive) joins two separate structural pieces called adherends. Adhesive bonding is used instead of, or in parallel with mechanical means of joining such as riveting or bolting.