Plywood wood based composites using crystalline/amorphous PLA polymer adhesives

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Abstract

Tailored crystalline and amorphous powdered polylactic acid (PLA) bio-renewable green polymers were studied for their use as adhesive for the joining of 2 and 3-ply veneers to produce and test plywood panels for strength, substrate failure and water soak resistance properties per ANSI/HPVA standards. For 3-ply construction Douglas Fir (DF) was used for the core whereas, Southern Pine (SP) veneer was used for the faces per sponsor specifications. It was found that heat or ionizing radiation tailored crystalline and amorphous type PLA-based adhesive resin formulations can both offer adequate strength as measured by the attained shear stress [ranging through ~3.4 MPa (500 psi)], and degree of substrate failure (50–90%) for a range of adhesive loadings ranging from 85 g/m2 to 300 g/m2. Highest shear strength values approaching 500 psi were obtained with emulsion spray based application using amorphous PLA in the 125–250 μm range. For 2-ply samples prepared with either crystalline or amorphous PLA adhesive, both offered satisfactory strength and soak resistance. Successful dry shear strength and 3-cycle soak water resistance under standard conditions were possible to achieve with amorphous PLA formulation for both 2-ply and 3-ply plywood samples. 3-ply tests with panels using crystalline form PLA adhesive formulation did not meet 3-cycle resistance metrics even with 300 g/m2 adhesive loading; although, 1-cycle soak resistance was possible. The superior performance of the amorphous formulation is deemed to arise out of improved wetting and substrate penetration due to its x10 higher melt index and x3 lower relative viscosity as compared with the crystalline form polymer adhesive. Plywood samples prepared with PLA formulations (as yet without cross-linking or strength tailoring), did not survive boiling water testing metrics as would be required of Type I (exterior grade) plywood. However, the research results indicate that ANSI/HVPA metrics for Type II (interior grade) plywood are attainable using the as-formulated amorphous form PLA based adhesive and the associated application methods described in the paper.

Introduction

The wood based composite (WBC) field spans the dimensional range of microns to meters [1]. Plywood component forms involve large sheets that are to be joined together [2]. Ahead of conducting research into more complex dispersion based systems such as particle boards, PLA technology (PLATech.) was assessed for its ability to offer a VOC-free, renewable, low-toxicity/flammability and bio-degradable potential alternative for joining plywood veneers in comparison to commonly used formaldehyde-based resins that are often toxic and flammable [[3], [4], [5]].

For this study methods for application were developed for which performance testing was conducted using the American National Standards Institute (ANSI) specified approaches developed for the Hardwood Plywood & Veneer Association (HPVA). At the time the research presented in this paper was conducted the acting standard was ANSI/HPVA HP-1-2009 (later updated in 2016) [6].

Since PLA can significantly hydrolyze and lose strength and functionality (under high temperatures and water content, e.g., under boiling conditions) as a primary goal for this study we decided to study its properties for use in so-called Type II interior use plywood manufacture. Per ANSI/HPVA HP-1-2009 Type II plywood must still pass two sets of primary performance tests, dry shear strength and resistance to delamination under three cycle soak tests in water under standard temperature and pressure (STP) conditions. The key difference for Type II vs Type I is the lack of testing under boiling water conditions (used as a success metric for plywoods used for exterior and marine use applications). Type II is considered water resistant vs. waterproof for Type I. The subsequent sections discuss the testing requirements, the approaches developed for assessing two different forms of PLA polymers and the results of tests.

Due to widespread use of British units in the U.S.A., discussion and results are presented in a mixed SI-British unit manner - mainly for ease of connection to published works and standards.

Section snippets

ASTM/HVPA type II plywood metrics

The ASTM/HVPA standards requires one to abide by their guidelines for sample preparation and testing for dry shear strength per ASTM D906 [[6], [7]]. The metrics are basically in two key categories:

PLA resin adhesive types and formulation, application and test sample preparation

PLA is a corn-derived, VOC-free renewable polymer [8] which makes it attractive from an environmental viewpoint. However, fundamentally, in order to prevail as a viable option for industry adoption, the challenge for PLA as an adhesive for plywood manufacture is to remain economical in competition with conventional adhesives – this is in addition to having met-exceeded the technical metrics per ASTM/HVPA standards. Consequently, a process must be devised to produce the PLA adhesive

Methods and modes of application of PLA-4043D-30C and PLA-10361D

In order to provide for as uniform a coverage as possible direct dispensing from a glue gun was not deemed appropriate – rather, it was determined that powder particulates would offer better performance. Being insoluble in water and not desiring to include organic (VOC-bearing) chemicals two methods were developed: (a) Dry application; and, (b) Emulsion spray-based application.

Results

The test specimens were subject to testing for dry shear and water soak attack per the ASTM standard guidelines. Results are presented in this section in two sub-sections.

Discussion on potential underlying contributors governing performance

It was revealing to note that both PLA-4043D-30C and PLA-10361D adhesive formulations dispensed in similar fashion over the same substrate veneers both offered similar (high enough) dry shear strength and substrate failure to meet/exceed the ASTM/HVPA metrics – even at the low end of 100 g/m2 loading levels. However, while PLA-10361D loaded panels loaded at/above 150 g/m2 performed well without delamination during the 3-cycle soak-dry tests, PLA-4043D-30C did not offer similar enablement for

Summary & conclusions

This paper covers research concerning use of two PLA polymer-based adhesive formulations based on crystalline (PLA-4043D-30C) and amorphous (PLA-10361D) resins, and their use for preparing and testing 2-ply and 3-ply plywood samples per the ASTM/HVPA standards. Two methods were developed and vetted for application of the adhesive: dry powder and wet spray modes. For dry powder application sieved powders in the 0.25–1.2 mm range were used. For wet spray based application a stable emulsion form

Acknowledgments

The authors are grateful for the research work which was sponsored in part by Purdue University, State of Indiana, USA and by Sagamore Adams Laboratories, LLC, USA, which also enabled testing for water soak performance and arranged for supply of plywood veneers from industry. The assistance provided by laboratory colleagues A. Hagen, K. Fisher, T. Grimes, N. Kostry, B. Archambault, and N. Hume is appreciated. Also acknowledged are the staff of Purdue University's Radiological and Environmental

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